Move safe_strerror to common/

This patch moves safe_strerror from the gdb/{posix,mingw}-hdep.c files
to the respective common/{posix,mingw}-strerror.c files.  This is a
preparation for the next patch, which shares a common code (to disable
address space randomization when creating a new inferior).

The patch has been regtested on Fedora 20 x86_64, and no regressions
were found.

gdb/ChangeLog
2015-01-15  Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (ALLDEPFILES): Including common/mingw-strerror.c and
	common/posix-strerror.c.
	(posix-strerror.o): New rule.
	(mingw-strerror.o): Likewise.
	* common/common-utils.h (safe_strerror): Move prototype to here,
	from utils.h.
	* common/common.host: New file.
	* common/mingw-strerror.c: Likewise.
	* common/posix-strerror.c: Likewise.
	* configure: Regenerated.
	* configure.ac: Source common/common.host.  Add variable
	common_host_obs to gdb_host_obs.
	* contrib/ari/gdb_ari.sh: Mention gdb/common/mingw-strerror.c and
	gdb/common/posix-strerror.c when warning about the use of
	strerror.
	* mingw-hdep.c (safe_strerror): Remove definition; move it to
	common/mingw-strerror.c.
	* posix-hdep.c (safe_strerror): Remove definition; move it to
	common/posix-hdep.c.
	* utils.h (safe_strerror): Remove prototype; move to
	common/common-utils.h.

gdb/gdbserver/ChangeLog
2015-01-15  Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (posix-strerror.o): New rule.
	(mingw-strerror.o): Likewise.
	* configure: Regenerated.
	* configure.ac: Source file ../common/common.host.  Initialize new
	variable srv_host_obs.  Add srv_host_obs to GDBSERVER_DEPFILES.

gdb/configure

@@ -5308,6 +5308,10 @@ fi
 
 . $srcdir/configure.host
 
+# Add in the common host objects.
+. $srcdir/common/common.host
+gdb_host_obs="$gdb_host_obs $common_host_obs"
+
 # Accumulate some settings from configure.tgt over all enabled targets
 
 TARGET_OBS=