--target: windows, linux, arm64, macos or a triple, in the compiler written in Finch

Targets, the target's C compiler (FINCH_CC) for linking, C import for the target (--target to libclang, its
headers), the AAPCS64 and Microsoft x64 ways of passing structs by value (and the System V one),
dllexport for libraries on Windows, .exe names, and running the program through wine or qemu-aarch64.
std/ gets time_windows.fch and proc_windows.fch: a module has a version per system (name_windows.fch,
name_macos.fch, name_linux.fch) that wins over name.fch. stdin, stdout and stderr work where C makes them macros.

Run for real: all 27 programs under tests/run and tests/new pass on ARM64 Linux under qemu and on Windows
under Wine (MinGW), structs by value and callbacks included. macOS is only checked as far as clang accepts the
IR; there is no emulator for it here.

std/proc_windows.fch

@@ -0,0 +1,45 @@
+// Programs, for Windows. The same functions as proc.fch (that file is for Linux and macOS).
+import "stdlib.h"
+import "process.h"
+import "direct.h"
+import "time.h"
+
+fn run(str command) -> int {
+    return shell(command)
+}
+
+fn output(str command) -> str! {
+    tmp := proc_env("TEMP", ".") + "\\finch_out_" + str(getpid()) + "_" + str(int(time(null)))
+    code := shell(command + " > \"" + tmp + "\" 2>&1")
+    text := read_file(tmp) or ""
+    delete_file(tmp)
+    if code != 0 {
+        return error("'" + command + "' exited with code " + str(code) + ": " + text.trim())
+    }
+    return text
+}
+
+fn proc_env(str name, str fallback) -> str {
+    v := getenv(name)
+    if v.len == 0 {
+        return fallback
+    }
+    return v
+}
+
+fn env(str name, str fallback) -> str {
+    return proc_env(name, fallback)
+}
+
+fn pid() -> int {
+    return int(getpid())
+}
+
+fn cwd() -> str {
+    []char buf
+    for i in 0..4096 {
+        buf.push(char(0))
+    }
+    getcwd(buf.ptr, 4096)
+    return str(buf.ptr)
+}