Use gnulib's strerror_r on MinGW

Use gnulib's strerror_r on MinGW

There is no need to keep mingw-strerror around; we can just always use
the code from posix-strerror. The main reason we had that code, it
seems, is to handle winsock error codes, but gnulib's version
handles those.

Unfortunately the code can't be moved into common-utils.c because
libinproctrace.so uses common-utils but not gnulib.

gdb/ChangeLog:

2019-11-15  Christian Biesinger  <cbiesinger@google.com>

	* Makefile.in: Replace {posix,mingw}-strerror.c with safe-strerror.c.
	* configure: Regenerate.
	* configure.ac: Don't source common.host.
	* gdbsupport/common.host: Remove.
	* gdbsupport/mingw-strerror.c: Remove.
	* gdbsupport/posix-strerror.c: Rename to...
	* gdbsupport/safe-strerror.c: ...this.

gdb/gdbserver/ChangeLog:

2019-11-15  Christian Biesinger  <cbiesinger@google.com>

	* Makefile.in: Add safe-strerror.c.
	* configure: Regenerate.
	* configure.ac: Don't source common.host.

Change-Id: I9e6d8a752fc398784201f370cafee65e0ea05474