Fix breakages in Linux-only build modes

- compile.com now polyfills -march=native which gcc/clang removed
- Guarantee zero Windows code is linked into non-Windows binaries
- MODE=tinylinux binaries are now back to being as tiny as ~4kb
- Improve the runtime's stack allocation / alignment hack
- GitHub Actions now tests Linux modes for assurance

libc/calls/clock_nanosleep.c

@@ -55,8 +55,10 @@ static errno_t sys_clock_nanosleep(int clock, int flags,
     rc = sys_clock_nanosleep_xnu(clock, flags, req, rem);
   } else if (IsOpenbsd()) {
     rc = sys_clock_nanosleep_openbsd(clock, flags, req, rem);
-  } else {
+  } else if (IsWindows()) {
     rc = sys_clock_nanosleep_nt(clock, flags, req, rem);
+  } else {
+    rc = enosys();
   }
   if (rc == -1) {
     rc = errno;

libc/calls/getpriority.c

@@ -25,6 +25,7 @@
 #include "libc/intrin/describeflags.internal.h"
 #include "libc/intrin/strace.internal.h"
 #include "libc/limits.h"
+#include "libc/sysv/errfuns.h"
 
 /**
  * Returns nice value of thing.
@@ -72,8 +73,10 @@ int getpriority(int which, unsigned who) {
       errno = rc;
       rc = -1;
     }
-  } else {
+  } else if (IsWindows()) {
     rc = sys_getpriority_nt(which, who);
+  } else {
+    rc = enosys();
   }
 #else
   rc = sys_getpriority(which, who);