build: fix four things a clean build found, three of them mine

`make` had been failing since the e1000 work and I did not see it, because I
was checking build output with `grep error` -- which matches neither "Błąd" nor
mkfs's "image full". Checking the exit status instead found all of this in a
minute.

**usbstick.img was full.** `make ldls-stage` puts a module and its BTF in
sysroot/lib/modules, and a vmlinux BTF is 5 MB on its own; the 16 MB test image
could not hold it. The size cannot move -- tools/windows/bootcheck.ps1 asserts
READ CAPACITY returns exactly 32768 sectors -- so the exclusions grew instead:
lib and usr as well as bin. That image exists to prove the USB driver can read
one block, so its contents were never the point. It also stopped a GPL module
riding into a test artifact, which RAMDISK_EXCLUDE does for the ramdisk and
nothing was doing for this one.

**toybox's httpd was shadowing ours.** /bin/httpd answered `-p` with toybox's
"Unknown option 'p'", because whichever install rule ran second won the name.
TOYBOX_SKIP existed for exactly this and held one entry, `ping`, found the same
hard way. It is now generated from cmd/ -- a list you have to remember is a
list that is wrong the next time cmd/ grows, and it already was: `chvt` had
been silently toybox's, whose VT ioctls are Linux's and not ours.

**ldlsd would not link with clang.** `*out = in;` on a 96-byte struct is a call
to memcpy as far as the C standard is concerned; gcc unrolled it and clang
emitted the call, so a freestanding program linked for months and then did not
on a machine where nothing about it had changed but the compiler. lib/glue.c
now provides memcpy, memmove, memset and memcmp -- the four an implementation
may call without being asked.

**OVMF was never found on NixOS.** `$(dir $(QEMU))share` is the *bin*
directory plus "share", so it looked in /usr/bin/share. The absolute paths
after it covered that up on distributions that keep firmware in /usr; on one
that keeps nothing there it meant the firmware was undiscoverable on a machine
that had it, and the only way to build was to copy the file in by hand. It
resolves the prefix properly now and looks in <prefix>/share/qemu too.

Verified by exit status, from a clean tree and a no-op: both 0. ldls-check
passes 27. The guest boots, serves /srv to the host and answers 403 to
%2e%2e/etc/passwd.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2

lib/glue.c

@@ -16,6 +16,71 @@ exit(void)
 /* _start lives in lib/start.S — it must read incoming registers, which
  * cannot be expressed portably in C. See that file. */
 
+// The four the compiler is allowed to call without being asked.
+//
+// A freestanding build gets no libc, but it does not get a compiler that
+// promises never to *call* one: the C standard lets an implementation emit
+// calls to memcpy, memmove, memset and memcmp for things that look nothing
+// like a function call. A struct assignment is the common one --
+//
+//     *out = in;                  /* struct pciuser_info, 96 bytes */
+//
+// -- which gcc unrolled into stores and clang turned into `call memcpy`, so
+// ldlsd linked for months and then did not, on a machine where nothing about
+// it had changed except which compiler ran. The fix is not to avoid struct
+// assignment; it is to provide the four functions, because the compiler is
+// within its rights either way.
+//
+// Deliberately simple. These are not the hot path of anything here, and a
+// clever one risks being turned back into a call to itself.
+void *
+memcpy(void *dst, const void *src, unsigned long n)
+{
+    unsigned char *d = dst;
+    const unsigned char *s = src;
+    while (n--) *d++ = *s++;
+    return dst;
+}
+
+void *
+memmove(void *dst, const void *src, unsigned long n)
+{
+    unsigned char *d = dst;
+    const unsigned char *s = src;
+
+    if (d == s || n == 0)
+        return dst;
+    // Backwards when they overlap the wrong way, which is the whole reason
+    // this is not memcpy.
+    if (d < s) {
+        while (n--) *d++ = *s++;
+    } else {
+        d += n; s += n;
+        while (n--) *--d = *--s;
+    }
+    return dst;
+}
+
+void *
+memset(void *dst, int c, unsigned long n)
+{
+    unsigned char *d = dst;
+    while (n--) *d++ = (unsigned char)c;
+    return dst;
+}
+
+int
+memcmp(const void *a, const void *b, unsigned long n)
+{
+    const unsigned char *p = a, *q = b;
+    while (n--) {
+        if (*p != *q)
+            return *p - *q;
+        p++; q++;
+    }
+    return 0;
+}
+
 void
 __stack_chk_fail(void)
 {