Use LOCAL_LDFLAGS_64 instead of enumerating 64-bit architectures

LOCAL_LDFLAGS_<arch> was being set for arm64, mips64, and x86_64.
Use LOCAL_LDFLAGS_64 instead.

Change-Id: Ib87855e8a7783f82461b707fffa7f1663e6be9c8
diff --git a/libc/Android.mk b/libc/Android.mk
index c6a5066..0fca92d 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -1164,9 +1164,7 @@
 # We'd really like to do this for all architectures, but since this wasn't done
 # before, these symbols must continue to be exported on LP32 for binary
 # compatibility.
-LOCAL_LDFLAGS_arm64 := -Wl,--exclude-libs,libgcc.a
-LOCAL_LDFLAGS_mips64 := -Wl,--exclude-libs,libgcc.a
-LOCAL_LDFLAGS_x86_64 := -Wl,--exclude-libs,libgcc.a
+LOCAL_LDFLAGS_64 := -Wl,--exclude-libs,libgcc.a
 
 $(eval $(call patch-up-arch-specific-flags,LOCAL_CFLAGS,libc_common_cflags))
 $(eval $(call patch-up-arch-specific-flags,LOCAL_SRC_FILES,libc_arch_dynamic_src_files))