libffi: Fix MIPS r6 support

for some instructions, MIPS r6 uses different encoding other than
the previous releases.

1. mips/n32.S disable .set mips4: since it casuses old insn encoding
   is used.
   https://github.com/libffi/libffi/pull/396 has been accepted as:
     94c102aa69b04337f63498e0e6551fcdce549ae5
2. mips/ffi.c: the encoding for JR is hardcoded: we need to use
   different value for r6 and pre-r6.
   https://github.com/libffi/libffi/pull/401 has been accpeted as:
     746dbe3a6a79a41931c03b51df2972be4d5e5028

libffi/
	PR libffi/83636
	* src/mips/n32.S: disable .set mips4
	* src/mips/ffi.c: use different JR encoding for r6.