libffi: mips/n32.S: disable .set mips4 on mips r6

[YunQiang Su]

https://github.com/libffi/libffi/pull/396

MIPS r6 drops some insn and change some insn's encoding,
so we need to drop .set mips4 when we build libffi for mips r6.

[YunQiang Su]

and a new pull request
https://github.com/libffi/libffi/pull/401

in mips/ffi.c, the encoding of jr also need to be fixed for r6.

Post by YunQiang Su
https://github.com/libffi/libffi/pull/396
MIPS r6 drops some insn and change some insn's encoding,
so we need to drop .set mips4 when we build libffi for mips r6.