Move gdb-dlfcn.[ch] to gdbsupport/

Move gdb-dlfcn.[ch] to gdbsupport/

I need to use 'gdb_dlopen' inside 'gdbsupport/', but it's not yet
supported there.  This commit moves 'gdb-dlfcn.[ch]' to 'gdbsupport/',
which makes it available also on gdbserver.

gdb/ChangeLog:
2019-08-23  Sergio Durigan Junior  <sergiodj@redhat.com>

	* configure.ac: Don't check for 'dlfcn.h' (moved to
	gdbsupport/common.m4).
	* Makefile.in (COMMON_SFILES): Move 'gdb-dlfcn.c' to
	'gdbsupport/'.
	(HFILES_NO_SRCDIR): Likewise, for 'gdb-dlfcn.h'.
	* compile/compile-c-support.c: Include
	'gdbsupport/gdb-dlfcn.h'.
	* gdbsupport/common.m4: Check for 'dlfcn.h'.
	* gdb-dlfcn.c: Move to...
	* gdbsupport/gdb-dlfcn.c: ... here.
	* gdb-dlfcn.h: Move to...
	* gdbsupport/gdb-dlfcn.h: ... here.

gdb/gdbserver/ChangeLog:
2019-08-23  Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (SFILES): Add 'gdbsupport/gdb-dlfcn.c'.
	(OBS): Add 'gdbsupport/gdb-dlfcn.o'.
	* config.in: Regenerate.
	* configure: Regenerate.