sh: add AT_HWCAP flag for J-Core cas.l instruction

The J-Core cpu has, as an ISA extension, an atomic compare-and-swap
instruction cas.l which applications need to use (instead the imask or
gusa atomic models, which are fundamentally limited to UP) for
synchronization in order to be compatible with SMP systems. Provide a
hwcap flag so that it's possible to do runtime selection and support
both.

Signed-off-by: Rich Felker <dalias@libc.org>