builtin,thirdparty: fix compilation of libgc with -cc msvc -gc boehm (thanks to @Ekopalypse)

thirdparty/libgc/include/gc.h

@@ -1,6 +1,8 @@
 /* This file is installed for backward compatibility. */
 #include "gc/gc.h"
 
+#ifndef _MSC_VER
 __attribute__ ((weak)) GC_API void GC_CALL GC_noop1_ptr(volatile void *p) {
    GC_noop1((u64)p);
 }
+#endif

vlib/builtin/builtin_d_gcboehm.c.v

@@ -114,7 +114,12 @@ $if gcboehm_leak ? {
 	#flag -DGC_DEBUG=1
 }
 
+$if windows && msvc {
+	#flag ucrtd.lib
+}
+
 #include <gc.h>
+
 // #include <gc/gc_mark.h>
 
 // replacements for `malloc()/calloc()`, `realloc()` and `free()`