xtensa: fix a6 and a7 handling in fast_syscall_xtensa
xtensa: fix a6 and a7 handling in fast_syscall_xtensa

Remove restoring a6 on some return paths and instead modify and restore
it in a single place, using symbolic name.
Correctly restore a7 from PT_AREG7 in case of illegal a6 value.

Cc: 4fbacc2fa0ffdbb11bf1ad6925b886ebd08dd15f@vger.kernel.org
Signed-off-by: Max Filippov <8c5ea195856807f4f02a6e0749c87ea6b7375f38@gmail.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs