gdb, gdbserver, gdbsupport: use AC_CANONICAL_{BUILD,HOST,TARGET} instead of AC_CANONI...

gdb, gdbserver, gdbsupport: use AC_CANONICAL_{BUILD,HOST,TARGET} instead of AC_CANONICAL_SYSTEM

`autoreconf -Wall` notes that AC_CANONICAL_SYSTEM is obsolete:

    configure.ac:36: warning: The macro `AC_CANONICAL_SYSTEM' is obsolete.

Replace it by AC_CANONICAL_BUILD, AC_CANONICAL_HOST and
AC_CANONICAL_TARGET in configure.ac files in gdb, gdbserver and
gdbsupport.  All three macros may not be needed everywhere, but it is
hard to completely audit the configure files to see which are required,
so I think it's better (and that there's no downside) to just call all
three.

gdb/ChangeLog:

	* configure.ac: Use AC_CANONICAL_{BUILD,HOST,TARGET} instead of
	AC_CANONICAL_SYSTEM.
	* configure: Re-generate.

gdbserver/ChangeLog:

	* configure.ac: Use AC_CANONICAL_{BUILD,HOST,TARGET} instead of
	AC_CANONICAL_SYSTEM.
	* configure: Re-generate.

gdbsupport/ChangeLog:

	* configure.ac: Use AC_CANONICAL_{BUILD,HOST,TARGET} instead of
	AC_CANONICAL_SYSTEM.
	* configure: Re-generate.

Change-Id: Ifd0e21f1e478634e768b5de1b8ee06a7f690d863