s390/vdso: revise CFI annotations of vDSO functions

Revise and add CFI CFA and register rule annotations to the vDSO
functions for proper stack unwinding and debugging.

Because glibc might call the vDSO in special ways, the vDSO code
does not rely on a stack frame created by the caller. The TOD clock
value can be therefore not stored in the pre-allocated stack area
and additional stack space is required.
To correctly annotate these situations with CFI, the .cfi_val_offset
directive is required to create relative offsets on the value of the
stack register %r15. Because the .cfi_val_offset directive is
available with recent GNU assembler versions only, additional checks
are necessary.

Note that if the vDSO is assembled with an older assembler version,
stack unwinding and debugging from within the vDSO code might not
be possible.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

s390/kernel: emit CFI data in .debug_frame and discard .eh_frame sections

Using perf probe and libdw on kernel modules failed to find CFI
data for symbols. The CFI data is stored in the .eh_frame section.
The elfutils libdw is not able to extract the CFI data correctly,
because the .eh_frame section requires "non-simple" relocations
for kernel modules.

The suggestion is to avoid these "non-simple" relocations by emitting
the CFI data in the .debug_frame section. Let gcc emit respective
directives by specifying the -fno-asynchronous-unwind-tables option.

Using the .debug_frame section for CFI data, the .eh_frame section
becomes unused and, thus, discard it for kernel and modules builds

The vDSO requires the .eh_frame section and, hence, emit the CFI data
in both, the .eh_frame and .debug_frame sections.

See also discussion on elfutils/libdw bugzilla:
https://sourceware.org/bugzilla/show_bug.cgi?id=22452

Suggested-by: Mark Wielaard <mark@klomp.org>
Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>