gnu: gcc-4.7: On powerpc64le, fix /lib64 references.

gnu: gcc-4.7: On powerpc64le, fix /lib64 references.

Fixes: <https://bugs.gnu.org/46253>.

* gnu/packages/gcc.scm (gcc-4.7)[#:phases][pre-configure]: When the
gcc/config/rs6000 directory exists, replace "/lib64" with "/lib" in all files
within it.  Note that this fix will be picked up by any package that re-uses
the pre-configure phase from gcc-4.7 (e.g., all the usual gcc packages).
(make-libstdc++)[#:phases][fix-rs6000-libdir]: New phase, which does the same
as above.  It was necessary to duplicate the fix here because make-libstdc++
does not re-use the pre-configure phase from gcc-4.7.