powerpc64le: Remove test for GCC 6.2

powerpc64le: Remove test for GCC 6.2

The configure fragment for powerpc64le contains a test for the presence
of several compiler builtins and of the __float128 type, which are
provided by GCC 6.2 for powerpc64le.  Since this configure test was
added, the compiler version required to build glibc for powerpc64le was
different than that required for the other architectures.

Now that glibc requires GCC 6.2 globally (since commit ID 4dcbbc3b28aa),
this patch removes the powerpc64le-specific test.

Even tough the configure test checks for compiler features rather than
compiler version, the intent of the test was to stop build attempts at
early stages, if they had been configured with a too old compiler.  It
was not the intention of the test to detect compiler breakage (such as
the removal of the required compiler features in future GCC versions),
and glibc is not the place to test for compiler regressions, anyway.

Tested for powerpc64le with GCC 6.2 (built with build-many-glibcs.py).

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>