Resolve assembler complains when including loongarch_arch.h

Resolve assembler complains when including loongarch_arch.h

The assembler will complain when we include loongarch_arch.h in
an assembly file as following:

crypto/loongarch_arch.h: Assembler messages:
crypto/loongarch_arch.h:12: Fatal error: no match insn: extern	unsigned int OPENSSL_loongarch_hwcap_P

So, the sentence of `extern unsigned int OPENSSL_loongarch_hwcap_P`
should be guarded with "#ifndef __ASSEMBLER__".

Fixes #21838.

Signed-off-by: Min Zhou <zhoumin@loongson.cn>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21839)