Share gdb/environ.[ch] with gdbserver

Share gdb/environ.[ch] with gdbserver

We will need access to the environment functions when we share
fork_inferior between GDB and gdbserver, therefore we simply make the
API on gdb/environ.[ch] available on common/.  No extra adjustments
are needed to make it compile on gdbserver.

gdb/ChangeLog:
2017-03-07  Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (SFILES): Replace "environ.c" with
	"common/environ.c".
	(HFILES_NO_SRCDIR): Likewise, for "environ.h".
	* environ.c: Include "common-defs.h" instead of "defs.h.  Moved
	to...
	* common/environ.c: ... here.
	* environ.h: Moved to...
	* common/environ.h: ... here.

gdb/gdbserver/ChangeLog:
2017-03-07  Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (SFILES): Add "common/environ.c".
	(OBJS): Add "common/environ.h".