selftests/powerpc: Allow building without static libc

Currently exec-target.c is linked statically with libc, which on Fedora
at least requires installing an additional package (glibc-static).

If that package is not installed the build fails with:

    CC       exec_target
  /usr/bin/ld: cannot find -lc: No such file or directory
  collect2: error: ld returned 1 exit status

All exec_target.c does is call sys_exit, which can be done easily enough
using inline assembly, and removes the requirement for a static libc to
be installed.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240812094152.418586-1-maddy@linux.ibm.com