Share regcache function regcache_raw_read_unsigned

Share regcache function regcache_raw_read_unsigned

This patch is in preparation for software single step support on ARM in
GDBServer. It adds a new shared function regcache_raw_read_unsigned and
regcache_raw_get_unsigned so that GDB and GDBServer can use the same call
to fetch a raw register into an integer.

No regressions, tested on ubuntu 14.04 ARMv7 and x86.
With gdbserver-{native,extended} / { -marm -mthumb }

gdb/ChangeLog:

	* Makefile.in (SFILES): Append common/common-regcache.c.
	(COMMON_OBS): Append common/common-regcache.o.
	(common-regcache.o): New rule.
	* common/common-regcache.h (register_status) New enum.
	(regcache_raw_read_unsigned): New declaration.
	* common/common-regcache.c: New file.
	* regcache.h (enum register_status): Move to common-regcache.h.
	(regcache_raw_read_unsigned): Likewise.
	(regcache_raw_get_unsigned): Likewise.

gdb/gdbserver/ChangeLog:

	* Makefile.in (SFILES): Append common/common-regcache.c.
	(OBS): Append common-regcache.o.
	(common-regcache.o): New rule.
	* regcache.c (init_register_cache): Initialize cache to
	REG_UNAVAILABLE.
	(regcache_raw_read_unsigned): New function.
	* regcache.h (REG_UNAVAILABLE, REG_VALID): Replaced by shared
	register_status enum.