--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/finch_win.h

@@ -0,0 +1,6 @@
+/* The Windows functions std/time_windows.fch needs. Declared here, not by including <windows.h>:
+   libclang can't parse that header with MinGW's intrinsics. Both are in kernel32. */
+unsigned long long GetTickCount64(void);
+void Sleep(unsigned long milliseconds);
+int QueryPerformanceCounter(long long *count);
+int QueryPerformanceFrequency(long long *frequency);