Build a Wasm libc.a from test_gen/build.rs

compiler/test_gen/src/helpers/dummy_libc_program.c

@@ -0,0 +1,8 @@
+#include <stdio.h>
+
+void main() {
+    printf("Hello, I am a C program and I use libc.\n");
+    printf("Would you like to use libc too, but just can't find the right path?\n");
+    printf("Well, simply compile me from build.rs using Zig's --global-cache-dir,\n");
+    printf("and have libc.a delivered right to your build directory!\n");
+}