powerpc64le: Check for compiler features for float128

On powerpc64le, support for float128 will be enabled, which requires some
compiler features to be present.  This patch adds a configure test to check
for such features, which are provided for powerpc64le since GCC 6.2.

Tested for powerpc64 and powerpc64le.

	* INSTALL: Regenerate.
	* manual/install.texi (Recommended Tools for Compilation): Mention
	the powerpc64le-specific requirement in the manual.
	* sysdeps/powerpc/powerpc64le/configure.ac: New file with checks
	for the compiler features required for building float128.
	* sysdeps/powerpc/powerpc64le/configure: New, auto-generated file.