Move store/extract ... integer routines to gdb/common/

In preparation for gdbserver using them as well.

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

	* Makefile.in (SFILES): Add common/gdb-byteswap.c.
	(HFILES_NO_SRCDIR): Add common/gdb-byteswap.h.
	(COMMON_OBS): Add common/gdb-byteswap.o.
	(gdb-byteswap.o): New rule.
	* common/common-types.h: Include bfd-types.h.
	* common/gdb-byteswap.c: New file.
	* common/gdb-byteswap.h: New file.
	* defs.h (extract_signed_integer, extract_unsigned_integer)
	(extract_long_unsigned_integer, extract_typed_address)
	(store_signed_integer, store_unsigned_integer): Moved to
	common/gdb-byteswap.h.
	* findvar.c (extract_signed_integer, extract_unsigned_integer)
	(extract_long_unsigned_integer, extract_typed_address)
	(store_signed_integer, store_unsigned_integer): Moved to
	common/gdb-byteswap.c.