stdlib.h: Fix qsort_r compatibility with GCC 12.

[jhb]

Summary

GCC 12 (unlike GCC 9) does not match a function argument passed to the
old qsort_r() API (as is used in the qsort_r_compat test) to a
function pointer type via __generic. It treats the function type as a
distinct type from a function pointer. As a workaround, add a second
definition of qsort_r for GCC 12 which uses the bare function type.

[jhb]

Ping?

[jhb]

This breaks the build with clang as clang says a _Generic selector can only have an object type and a function pointer type isn't valid. GCC 9 was happy with the existing version, so this might be some regression in GCC 10, 11, or 12.

[jhb]

This is a bit of a stop-gap. I probably should create a smaller reproducible test case to submit as a bug upstream to GCC.

include/stdlib.h

355

I haven't tested GCC 10 or 11.

[emaste]

LGTM as it applies only to GCC 12.
Would be great to have a small reproducer shared with upstream.