selftests/powerpc/dexcr: Add chdexcr utility

Adds a utility to exercise the prctl DEXCR inheritance in the shell.
Supports setting and clearing each aspect.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
[mpe: Use correct SPDX license, use execvp() for usability, print errors]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240417112325.728010-9-bgray@linux.ibm.com