Actually turn on -Werror for crtbegin.

And then -Wno-gcc-compat in the right place this time. (But leaving the
other one because I don't care in the non-crtbegin code either.)

Bug: N/A
Test: builds without warnings now
Change-Id: Id2cfa3392ee80c045644ebfdf92c6d341db90964

libc/Android.bp

@@ -1948,6 +1948,8 @@ cc_defaults {
 
     no_default_compiler_flags: true,
 
+    cflags: ["-Wno-gcc-compat", "-Werror"],
+
     arch: {
         arm: {
             local_include_dirs: ["arch-arm/include"],
@@ -1974,6 +1976,8 @@ cc_defaults {
 cc_defaults {
     name: "crt_so_defaults",
 
+    cflags: ["-Wno-gcc-compat", "-Werror"],
+
     vendor_available: true,
     arch: {
         mips: {