libstdc++: Fix up abi.exp FAILs on powerpc64-linux

As discussed on IRC, my _Float128/_Float64x support changes broke
abi.exp testing on powerpc64-linux.

The
_ZTIDF128_@@CXXABI_1.3.14
_ZTIDF64x@@CXXABI_1.3.14
_ZTIPDF128_@@CXXABI_1.3.14
_ZTIPDF64x@@CXXABI_1.3.14
_ZTIPKDF128_@@CXXABI_1.3.14
_ZTIPKDF64x@@CXXABI_1.3.14
symbols only appear on powerpc64le-linux (both when building against
very old glibcs as well as contemporary glibcs), while they don't
appear on powerpc64-linux, because the latter never has _Float128 or
_Float64x support.

But we were using the same baseline_symbols.txt file for both
powerpc64-linux and powerpc64le-linux, even when it contained quite a lot
of stuff specific to the latter; but that was just the IEEE128 related
stuff that appears only when configured against not very old glibc.

The following patch keeps those exports as is and just splits the
config/abi/post/ files, copies the current one to powerpc64le-linux
unmodified and removes the above mentioned symbols plus all
GLIBCXX_IEEE128_3.4.{29,30,31} and CXXABI_IEEE128_1.3.13 symbols
from the powerpc64-linux version.

2023-05-03  Jakub Jelinek  <jakub@redhat.com>

	* configure.host (abi_baseline_pair): Use powerpc64le-linux-gnu
	rather than powerpc64-linux-gnu for powerpc64le*-linux*.
	* config/abi/post/powerpc64-linux-gnu/baseline_symbols.txt: Remove
	_ZTI*DF128_, _ZTI*DF64x symbols and symbols in
	GLIBCXX_IEEE128_3.4.{29,30,31} and CXXABI_IEEE128_1.3.13 symbol
	versions.
	* config/abi/post/powerpc64le-linux-gnu/baseline_symbols.txt: New
	file.