work around ppc-vx6 make limitations

libstdc++-v3 in gcc-15 gained uses of a make function that was added
in GNU make 3.81, but ppc-vx6 is GNU make 3.80, so calling the
function returns an empty string, and the build fails.

gcc/doc/install.texi states GCC requires make 3.80 to build, so
rewrite those bits to avoid the new function.

Install locations don't need abspath because they're already supposed
to be absolute, since installation can't work with relative paths and
DESTDIR is supposed to be prependable.

$? may gain the srcdir VPATH, which would complicate dropping abspath,
so iterate over the two filenames.


for  libstdc++-v3/ChangeLog

	* src/c++23/Makefile.am (libstdc++.modules.json): Rewrite
	without abspath make function.
	(stamp-modules-bits): Likewise.
	* src/c++23/Makefile.in: Rebuilt.
(cherry picked from commit b0812255e4212f14d07e2d9077d70ba491666e66)