LoongArch: config: Added HAVE_LOONGARCH_VEC_ASM.

This patch checks if assembler supports vector instructions to
generate LASX/LSX code or not, and then define HAVE_LOONGARCH_VEC_ASM macro

We have added support for vector instructions in binutils-2.41
See:
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=75b2f521b101d974354f6ce9ed7c054d8b2e3b7a

commit 75b2f521b101d974354f6ce9ed7c054d8b2e3b7a
Author: mengqinggang <mengqinggang@loongson.cn>
Date:   Thu Jun 22 10:35:28 2023 +0800

LoongArch: gas: Add lsx and lasx instructions support

gas/ChangeLog:

        * config/tc-loongarch.c (md_parse_option): Add lsx and lasx option.
        (loongarch_after_parse_args): Add lsx and lasx option.

opcodes/ChangeLog:

        * loongarch-opc.c (struct loongarch_ase): Add lsx and lasx
        instructions.