[S390] vdso: clock_gettime of CLOCK_THREAD_CPUTIME_ID with noexec=on

The combination of noexec=on and a clock_gettime call with clock id
CLOCK_THREAD_CPUTIME_ID is broken. The vdso code switches to the
access register mode to get access to the per-cpu data structure to
execute the magic ectg instruction. After the ectg instruction the
code always switches back to the primary mode but for noexec=on the
correct mode is the secondary mode. The effect of the bug is that the
user space program looses the access to all mappings without PROT_EXEC,
e.g. the stack. The problem is fixed by restoring the mode that has
been active before the switch to the access register mode.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>