valgrind: backport rseq work-around

valgrind: backport rseq work-around

Using glibc-2.35-alpha, valgrind ptests mostly all fail with the error:
   WARNING: unhandled amd64-linux syscall: 334
   You may be able to write your own handler.
With this patch applied qemux86-64 ptest results were:
   TOTAL: 728
   PASSED: 709
   FAILED: 0
   SKIPPED: 19

This patch implements the Linux rseq syscall as ENOSYS while
a proper fix is being worked on as desribed in the patch
commit log.

Signed-off-by: Randy MacLeod <Randy.MacLeod@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>