MIPS: Add speculation_barrier support

speculation_barrier for MIPS needs sync+jr.hb (r2+),
so we implement __speculation_barrier in libgcc, like arm32 does.

gcc/ChangeLog:
	* config/mips/mips-protos.h (mips_emit_speculation_barrier): New
	prototype.
	* config/mips/mips.cc (speculation_barrier_libfunc): New static
	variable.
	(mips_init_libfuncs): Initialize it.
	(mips_emit_speculation_barrier): New function.
	* config/mips/mips.md (speculation_barrier): Call
	mips_emit_speculation_barrier.

libgcc/ChangeLog:
	* config/mips/lib1funcs.S: New file.
	define __speculation_barrier and include mips16.S.
	* config/mips/t-mips: define LIB1ASMSRC as mips/lib1funcs.S.
	define LIB1ASMFUNCS as _speculation_barrier.
	set version info for __speculation_barrier.
	* config/mips/libgcc-mips.ver: New file.
	* config/mips/t-mips16: don't define LIB1ASMSRC as mips16.S
	included in lib1funcs.S now.