Fix compiling error for mips32r6 and mips64r6

There are some compiling errors for mips32r6 and mips64r6:

crypto/bn/bn-mips.S:56: Error: opcode not supported on this processor: mips2 (mips2) `mulu $1,$12,$7'
crypto/mips_arch.h: Assembler messages:
crypto/mips_arch.h:15: Error: junk at end of line, first unrecognized character is `&'

Signed-off-by: Hua Zhang <hua.zhang1974@hotmail.com>

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8464)