Fix gdbserver's regcache_raw_read_unsigned on big endian hosts

The regcache_raw_read_unsigned function is memcpy'ing a 32-bit value
directly into a 64-bit variable, which doesn't work on big endian
targets.

Fix this by memcpy'ing to a buffer, and then using
extract_unsigned_integer, just like gdb's version.

gdb/gdbserver/ChangeLog:
2016-02-11  Pedro Alves  <palves@redhat.com>

	* Makefile.in (SFILES): Add $(srcdir)/common/gdb-byteswap.c.
	(gdb-byteswap.o): New rule.
	* regcache.c: Include "gdb-byteswap.h".
	(host_bfd_endian): New function.
	(regcache_raw_read_unsigned): Use extract_unsigned_integer and
	host_bfd_endian.