mmix: Don't turn off .hidden support.

mmix: Don't turn off .hidden support.

In 2012 e2769e908a915ebbc/r192344, I added the following lines, that I
now delete.  I've changed my mind: ELF-related targets based on gas,
that support only static linking, have .hidden "for free", regardless
of the visibility of the hidden object in the linked executable.  No
regressions for mmix-knuth-mmixware and "fixes":

FAIL: gcc.dg/torture/pr94947-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  (test for excess errors)
Excess errors:
lto1: warning: visibility attribute not supported in this configuration; ignored [-Wattributes]

...a warning which is emitted for "all" LTO tests (of which most have
warnings filtered out); LTO "thinks" all supported targets have
.hidden support.

gcc:
	* configure.ac (out-of-tree linker .hidden support): Don't turn off
	for mmix-knuth-mmixware.
	* configure: Regenerate.