LoongArch: Explicitly set -fdirect-access-external-data for vmlinux

After this llvm commit [1], The -fno-pic does not imply direct access
external data. Explicitly set -fdirect-access-external-data for vmlinux
that can avoids GOT entries.

Link: https://github.com/llvm/llvm-project/commit/47eeee297775347cbdb7624d6a766c2a3eec4a59Suggested-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: WANG Rui <wangrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/Makefile

@@ -68,6 +68,7 @@ LDFLAGS_vmlinux			+= -static -n -nostdlib
 ifdef CONFIG_AS_HAS_EXPLICIT_RELOCS
 cflags-y			+= $(call cc-option,-mexplicit-relocs)
 KBUILD_CFLAGS_KERNEL		+= $(call cc-option,-mdirect-extern-access)
+KBUILD_CFLAGS_KERNEL		+= $(call cc-option,-fdirect-access-external-data)
 KBUILD_AFLAGS_MODULE		+= $(call cc-option,-fno-direct-access-external-data)
 KBUILD_CFLAGS_MODULE		+= $(call cc-option,-fno-direct-access-external-data)
 KBUILD_AFLAGS_MODULE		+= $(call cc-option,-mno-relax) $(call cc-option,-Wa$(comma)-mno-relax)