gcc: if isPower64 && !isMusl use --with-long-double-format=ieee

Use IEEE-standard floating point on `powerpc64le` instead of
IBM-proprietary formats.  The GCC wiki has more details on the history
here:

  https://gcc.gnu.org/wiki/Ieee128PowerPC

Nixpkgs' stdenv has no legacy `powerpc64le` installs to deal with
(stdenv did not bootstrap on powerpc64le until very recenty), so it's
much easier decision for us.

Red Hat (i.e. IBM) has tried to do this in each of the last *six*
releases:

  https://fedoraproject.org/wiki/Changes/PPC64LE_Float128_Transition

they and finally shipped it in May with Fedora 36:

  https://bugzilla.redhat.com/show_bug.cgi?id=1649936

Apparently glibc 2.35 fixes the last blocker.

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

pkgs/development/compilers/gcc/common/platform-flags.nix

@@ -10,7 +10,11 @@ in lib.concatLists [
   (lib.optional (p ? fpu) "--with-fpu=${p.fpu}")
   (lib.optional (p ? float) "--with-float=${p.float}")
   (lib.optional (p ? mode) "--with-mode=${p.mode}")
-  (lib.optional
-    (let tp = targetPlatform; in tp.isPower && tp.libc == "glibc" && tp.is64bit)
-    "--with-long-double-128")
+  (lib.optionals targetPlatform.isPower64
+    # musl explicitly rejects 128-bit long double on
+    # powerpc64; see musl/arch/powerpc64/bits/float.h
+    (lib.optionals (!targetPlatform.isMusl) [
+      "--with-long-double-128"
+      "--with-long-double-format=ieee"
+    ]))
 ]