sparc64: Remove unwind information from signal return stubs [BZ#31244]

Similar to sparc32 fix, remove the unwind information on the signal
return stubs.  This fixes the regressions:

FAIL: nptl/tst-cancel24-static
FAIL: nptl/tst-cond8-static
FAIL: nptl/tst-mutex8-static
FAIL: nptl/tst-mutexpi8-static
FAIL: nptl/tst-mutexpi9

On sparc64-linux-gnu.