sparc32: Add nop before __startcontext to stop unwinding [BZ #22919]

On sparc32 tst-makecontext fails, as backtrace called within a context
created by makecontext to yield infinite backtrace.

Fix that the same way than nios2 by adding a nop just before
__startcontext. This is needed as otherwise FDE lookup just repeatedly
finds __setcontext's FDE in an infinite loop, due to the convention of
using 'address - 1' for FDE lookup.

Changelog:
	[BZ #22919]
	* sysdeps/unix/sysv/linux/sparc/sparc32/setcontext.S (__startcontext):
	Add nop before __startcontext, add explaining comments.