xtensa: fix a6 and a7 handling in fast_syscall_xtensa

commit d1b6ba82a50cecf94be540a3a153aa89d97511a0 upstream.

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.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Zefan Li <lizefan@huawei.com>