PowerPC64 little-endian fixes: VSX tests and pseudo-regs

PowerPC64 little-endian fixes: VSX tests and pseudo-regs

Many VSX test were failing on powerpc64le-linux, since -as opposed to the
AltiVec tests- there never were little-endian versions of the test patterns.

This patch adds such patterns, along the lines of altivec-regs.exp.

In addition, there is an actual code change required: For those VSX
registers that overlap a floating-point register, the FP register
overlaps the most-significant half of the VSX register both on big-
and little-endian systems.  However, on little-endian systems, that
half is stored at an offset of 8 bytes (not 0).  This works already
for the "real" FP registers, but current code gets it wrong for
the "extended" pseudo FP register GDB generates for the second
half of the VSX register bank.

This patch updates the corresponding pseudo read/write routines
to take the appropriate offset into consideration.

gdb/ChangeLog:

	* rs6000-tdep.c (efpr_pseudo_register_read): Use correct offset
	of the overlapped FP register within the VSX register on little-
	endian platforms.
	(efpr_pseudo_register_write): Likewise.

gdb/testsuite/ChangeLog:

	* gdb.arch/vsx-regs.exp: Check target endianness.  Provide variants
	of the test patterns for use on little-endian systems.