fpu: save each process's x87/SSE registers; keep the kernel off them

Nothing ever saved the FPU and SSE registers on a context switch.
musl's memcpy, the compiler's struct copies and any arithmetic on
doubles keep live values in %xmm, so two processes sharing a CPU wrote
over each other's: two shells counting in a loop on one processor took
a NULL+8 page fault within seconds, at a different rip every time. It
reproduces on kernels from before this week too -- it had always been
there, and every crash nobody could explain was a candidate.

sched() now fxsaves the process leaving the CPU and scheduler()
fxrstors the one arriving. A new process starts from the ABI's state
(FCW 0x37F, MXCSR 0x1F80), fork copies the parent's live registers,
exec resets them, and a signal handler's are put back by rt_sigreturn
so the interrupted code finds what it had.

The kernel was free to use %xmm too -- GCC vectorised struct copies
and memset into 153 instructions of it -- so an interrupt could
clobber the registers of the program it interrupted with no switch at
all. Kernel, lwIP and uACPI objects are built -mgeneral-regs-only now
and the image has none; cmd/ keeps SSE.

Checked under QEMU: three concurrent loops three times over, on 1 CPU
and on 4, no faults; before, every run faulted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

include/x86.h

@@ -25,6 +25,19 @@ rdtsc(void)
   return ((uint64)hi << 32) | lo;
 }
 
+// The x87/MMX/SSE register file, to and from a 512-byte, 16-byte-aligned area.
+static inline void
+fxsave(void *area)
+{
+  asm volatile("fxsave64 (%0)" : : "r"(area) : "memory");
+}
+
+static inline void
+fxrstor(const void *area)
+{
+  asm volatile("fxrstor64 (%0)" : : "r"(area) : "memory");
+}
+
 static inline void
 insl(int port, void *addr, int cnt)
 {