Require GCC 6.2 or later to build glibc

[Gabriel F. T. Gomes]

On Thu, Jan 31 2019, Joseph Myers wrote:
> 
> Thus, this patch increases the minimum to 6.2, removing the
> documentation of the separate requirement for powerpc64le.  It does
> not remove the powerpc64le configure test, or any __GNUC_PREREQ that
> could be removed as not being in installed headers or files shared
> with gnulib; I think such cleanups are best done separately.

I'd like to keep the tests for powerpc64le requirements, namely, the
checks for the presence of quad-precision builtins, and POWER8 as target
(sysdeps/powerpc/powerpc64/le/configure.ac).  Was it your intention to
suggest that we should remove the check for the builtins or are these
cleanups about __GNUC_PREREQ alone?

The patch looks good to me.