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/47eeee297775347cbdb7624d6a766c2a3eec4a59
Suggested-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: WANG Rui <wangrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>