guix: disable libquadmath in GCC

Prunes:
libquadmath.a
libquadmath.la
libquadmath.so
libquadmath.so.0
libquadmath.so.0.0.0

contrib/guix/manifest.scm

@@ -440,6 +440,7 @@ inspecting signatures in Mach-O binaries.")
                   "--enable-cet=yes",
                   "--disable-gcov",
                   "--disable-libgomp",
+                  "--disable-libquadmath",
                   "--disable-libsanitizer",
                   building-on)))
         ((#:phases phases)