mips: Fix store/load gp registers to/from ucontext_t

mips: Fix store/load gp registers to/from ucontext_t

General purpose registers in mcontext_t structure
are 8 bytes long for both MIPS32/MIPS64.

get/set/make/swap context implementations for MIPS O32
incorrectly assume that general purpose registers
in this structure are 4 bytes long.

This patch is fixing that.

Tested for MIPS O32 LE and BE.
Compared objdump of modified functions for mips n32 and mips n64.

	[BZ #21548]
	* sysdeps/unix/sysv/linux/mips/getcontext.S: Define MCONTEXT_SZGREG as
	8 and use it when copying general purpose registers.
	* sysdeps/unix/sysv/linux/mips/makecontext.S: Likewise.
	* sysdeps/unix/sysv/linux/mips/mips32/Makefile: Include new test for
	mips o32.
	* sysdeps/unix/sysv/linux/mips/mips32/bug-getcontext-mips-gp.c: Added
	new test for mips o32.
	* sysdeps/unix/sysv/linux/mips/setcontext.S: Define MCONTEXT_SZGREG as
	8 and use it when copying general purpose registers.
	* sysdeps/unix/sysv/linux/mips/swapcontext.S: Likewise.