loongarch: Disable relaxation relocations

loongarch: Disable relaxation relocations

A working GRUB cannot be built with upcoming binutils and GCC, because linker
relaxation was added [1] causing new unsupported relocations to appear in modules.

So we pass -mno-relax to GCC if it is supported, to disable relaxation and make
GRUB forward-compatible with new toolchains.

While similar code already exists for sparc64 in configure.ac, sparc64 sets
LDFLAGS while LoongArch requires CFLAGS to be set. If we only set LDFLAGS on
LoongArch, GCC will still generate relaxation relocations in the .o files, so
the sparc64 code cannot be reused.

[1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=56576f4a722b7398d35802ecf7d4185c27d6d69b

Signed-off-by: Xiaotian Wu <wuxiaotian@loongson.cn>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>