Add --enable-ubsan

Add --enable-ubsan

This adds --enable-ubsan to gdb's configure.  By default it is enabled
in development mode, and disabled otherwise.  This passes both
-fsanitize=undefined and -fno-sanitize-recover=undefined to
compilations, so that undefined behavior violations will be sure to
cause test failures.

gdb/ChangeLog
2018-10-03  Tom Tromey  <tom@tromey.com>

	* README: Mention --enable-ubsan.
	* NEWS: Mention --enable-ubsan.
	* acinclude.m4: Include sanitize.m4.
	* configure: Rebuild.
	* configure.ac: Call AM_GDB_UBSAN.
	* sanitize.m4: New file.

gdb/doc/ChangeLog
2018-10-03  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Configure Options): Document --enable-ubsan.