Allow sys/auxv.h to be used from the testsuite on powerpc and sparc.

elf/tst-auxv.c includes misc/sys/auxv.h, which ends up not actually
being included due to the guard overlap, and getauxval becomes an
implicit declaration and implicit pointer conversion which means, at
best, the test isn't actually testing what it thinks it is and, at
worst, it'll crash and burn on platforms where implict pointer
conversion is a Very Bad Thing.

	* sysdeps/powerpc/bits/hwcap.h: Allow _SYSDEPS_SYSDEP_H guard as a
	synonym for _SYS_AUXV_H to allow direct inclusion.
	* sysdeps/sparc/bits/hwcap.h: Likewise.
	* sysdeps/powerpc/sysdep.h: Define _SYSDEPS_SYSDEP_H instead of
	_SYS_AUXV_H so we can include sysdep.h and sys/auxv.h together.
	* sysdeps/sparc/sysdep.h: Likewise.