Fix some sparc conform test failures in siginfo.h

	* sysdeps/unix/sysv/linux/sparc/bits/siginfo.h (EMT_TAGOVF): Protect
	with XOPEN cpp guards.
Fix some sparc -Wundef build warnings.

	* sysdeps/sparc/bits/string.h (_STRING_ARCH_unaligned): Define to
	0.