Disable the undefined behavior sanitizer by default

Disable the undefined behavior sanitizer by default

There have been a few undefined behavior failures reported, and Pedro
suggested that the sanitizer be disabled by default.  This patch
implements this.

gdb/ChangeLog
2018-10-09  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.
	* sanitize.m4 (AM_GDB_UBSAN): Default to no.
	* NEWS: Update --enable-ubsan documentation.

gdb/doc/ChangeLog
2018-10-09  Tom Tromey  <tom@tromey.com>

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