Annotate No_libcrt with arch_variant

libcrt is crashing libc on x86/x86_64 for unknown reason. This enables
us to disable libcrt for these architectures for libc for now.

Test: m checkbuild
Bug: 29275768
Change-Id: I5a717286b3d9cc1ba8cdcd33bafb9c225de4aff8
diff --git a/cc/linker.go b/cc/linker.go
index 649185a..25aedd0 100644
--- a/cc/linker.go
+++ b/cc/linker.go
@@ -61,7 +61,7 @@
 	No_libgcc *bool
 
 	// don't link in libclang_rt.builtins-*.a
-	No_libcrt *bool
+	No_libcrt *bool `android:"arch_variant"`
 
 	// Use clang lld instead of gnu ld.
 	Use_clang_lld *bool `android:"arch_variant"`