mips: Remove rt_sigreturn usage on context function

Similar to powerpc, mips also issues rt_sigreturn for setcontext
case the v0 value saved is not the one set by setcontext or
makecontext. As for powerpc, it is intention is no really supported
since setcontext is not async-signal-safe.

Checked the context tests on mips64-linux-gnu and mips-linux-gnu.

	* sysdeps/unix/sysv/linux/mips/getcontext.S (__getcontext): Remove
	the magic flag store.
	* sysdeps/unix/sysv/linux/mips/makecontext.S (__makecontext):
	Likewise.
	* sysdeps/unix/sysv/linux/mips/swapcontext.S (__swapcontext):
	Likewise.
	* sysdeps/unix/sysv/linux/mips/setcontext.S (__setcontext):
	Remove rt_sigreturn call.