add -Wno-pragma-pack when targeting windows-gnu

windows.h has files such as pshpack1.h which do #pragma packing,
triggering a clang warning. So for this target, this warning is
disabled.

this commit also improves the error message printed when no libc can be
used, printing the "zig triple" rather than the "llvm triple".

src/main.cpp

@@ -982,7 +982,7 @@ int main(int argc, char **argv) {
 
     if (target_requires_pic(&target, have_libc) && want_pic == WantPICDisabled) {
         Buf triple_buf = BUF_INIT;
-        get_target_triple(&triple_buf, &target);
+        target_triple_zig(&triple_buf, &target);
         fprintf(stderr, "`--disable-pic` is incompatible with target '%s'\n", buf_ptr(&triple_buf));
         return print_error_usage(arg0);
     }