LoongArch: Add Kconfig option AS_HAS_EXPLICIT_RELOCS

GNU as >= 2.40 and GCC >= 13 will support using explicit relocation
hints in the assembly code, instead of la.* macros. The usage of
explicit relocation hints can improve code generation so it's enabled
by default by GCC >= 13.

Introduce a Kconfig option AS_HAS_EXPLICIT_RELOCS as the switch for
"use explicit relocation hints or not".

Tested-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>