callfuncs.exp: avoid spurious register differences in sparc64 targets.

The Linux kernel disables the FPU upon returning to userland.  This
introduces spurious failures in the register preservation tests in
callfuncs.exp, since the pstate.PEF bit gets cleared after system
calls.

This patch filters out the pstate register in sparc64-*-linux-gnu
targets, so the relevant tests are no longer fooled and pass.

gdb/testsuite/ChangeLog:

2015-11-20  Jose E. Marchesi  <jose.marchesi@oracle.com>

        * gdb.base/callfuncs.exp (fetch_all_registers): Filter out the
          pstate register when comparing registers values in
          sparc64-*-linux-gnu targets to avoid spurious differences.