gdb: Powerpc mark xfail in gdb.base/catch-syscall.exp

Powerpc is not reporting the

  Catchpoint 1 (returned from syscall execve),  ....

as expected.  The issue appears to be with the kernel not returning the
expected result.  This patch marks the test failure as an xfail.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28623