S390: Implement 64-bit __fentry__

* Since __fentry__ is almost the same as _mcount, reuse the code by
  #including it twice with different #defines around.
* Remove LA usages - they are needed in 31-bit mode to clear the top
  bit, but in 64-bit they appear to do nothing.
* Add CFI rule for the nonstandard return register. This rule applies
  to the current function (binutils generates a new CIE - see
  gas/dw2gencfi.c:select_cie_for_fde()), so it is not necessary to put
  __fentry__ into a new file.
* Fix CFI offset for %r14.
* Add CFI rule for %r0.
* Fix unwound value of %r15 being off by 244 bytes.
* Unwinding in __fentry__@plt does not work, no plan to fix it - it
  would require asking linker to generate CFI for return address in
  %r0.  From functional perspective keeping it broken is fine, since
  the callee did not have a chance to do anything yet.  From
  convenience perspective it would be possible to enhance GDB in the
  future to treat __fentry__@plt in a special way.
* Fix whitespace.
* Fix offsets in comments, which were copied from 32-bit code.
* 32-bit version will not be implemented, since it's not compatible
  with the corresponding PLT stubs: they assume %r12 points to GOT,
  which is not the case for gcc-emitted __fentry__ stub, which runs
  before the prolog.

This patch adds the runtime support in glibc for the -mfentry
gcc feature introduced in [1] and [2].

[1] https://gcc.gnu.org/ml/gcc-patches/2018-07/msg00784.html
[2] https://gcc.gnu.org/ml/gcc-patches/2018-07/msg00912.html

ChangeLog:

	* sysdeps/s390/s390-64/Versions (__fentry__): Add.
	* sysdeps/s390/s390-64/s390x-mcount.S: Move the common
	code to s390x-mcount.h and #include it.
	* sysdeps/s390/s390-64/s390x-mcount.h: New file.
	* sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist
	(__fentry__): Add.

0 files changed, 0 insertions, 0 deletions