x86: tls prevent_tail_call

Fix a kernel bug (vmware boot problem) reported by Tomasz Grobelny,
which occurs with certain .config variants and gccs.

The x86 TLS cleanup in commit efd1ca52


made the sys_set_thread_area and sys_get_thread_area functions ripe for
tail call optimization.  If the compiler chooses to use it for them, it
can clobber the user trap frame because these are asmlinkage functions.

Reported-by: Tomasz Grobelny <tomasz@grobelny.oswiecenia.net>
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>