Fortran: Re-enable 128-bit integers for AMD GCN

This patch reverts the part of Tobias's patch for PR target/96306 that
disables 128-bit integer support for AMD GCN.

2021-06-18  Julian Brown  <julian@codesourcery.com>

libgfortran/
	PR target/96306
	* configure.ac: Remove stanza that removes KIND=16 integers for AMD GCN.
	* configure: Regenerate.