Run check-installed-headers concurrently for each header

Run check-installed-headers concurrently for each header

The check-installed-headers-c/-cxx tests ran one script invocation per
subdirectory over all of its installed headers, performing about 80
compiler invocations per header serially.

Give each header its own intermediate target so the compiler
invocations parallelize under the make jobserver, recording the
per-header script exit status next to the output.  The .out target
concatenates the per-header outputs in the original $(headers) order
and fails if any recorded status is non-zero, so both the .out contents
(verified byte-identical for all 76 files) and the tests.sum results
are unchanged.

Results on a x86_64 machine [1] from a make check with run-built-tests=no
show neutral results, and on aarch64 machine [2] it improves from 241.574s
to 182.405.

[1] Ryzen 5900x, 12c/24t, gcc 16.1.1, binutils 2.26, and GNU make 4.3
[2] N1, 80c, gcc 15.1.1, binutils 2.25, GNU make 4.3

Reviewed-by: Sam James <sam@gentoo.org>