linux: Also check pkey_get for ENOSYS on tst-pkey (BZ 31996)

The powerpc pkey_get/pkey_set support was only added for 64-bit [1],
and tst-pkey only checks if the support was present with pkey_alloc
(which does not fail on powerpc32, at least running a 64-bit kernel).

Checked on powerpc-linux-gnu.

[1] https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=a803367bab167f5ec4fde1f0d0ec447707c29520


Reviewed-By: Andreas K. Huettel <dilfridge@gentoo.org>