s390: Store SFrame CFA offset adjusted and scaled down

s390: Store SFrame CFA offset adjusted and scaled down

In SFrame V2 the size of the offsets following an SFrame FRE can be
either signed 8-bit, 16-bit, or 32-bit integer, with the largest offset
determining their size:
  1. CFA offset from CFA base register
  2. RA (stack save slot) offset from CFA, usually -48 on s390x if saved
  3. FP (stack save slot) offset from CFA, usually -72 on s390x if saved
The FP and RA offsets from CFA, when FP/RA saved on the stack, usually
have fixed values that fit into signed 8-bit SFrame offsets.  Likewise
the DWARF register numbers on s390x of general registers (GR; 0-15) and
floating-point registers (FPR; 16-31), when FP/RA saved in registers.
With that the CFA offset from CFA base register has the greatest impact
on the signed SFrame offset size.

The s390x ELF ABI defines the stack pointer (SP) to be 8-byte aligned
[1] and the CFA as SP at call site + 160 [2].  The CFA offset from CFA
base register is therefore always a multiple of 8.

On s390x store the SFrame CFA offset from CFA base register scaled down
by the s390x-specific CFA alignment factor of 8, in addition to the
adjustment by the s390x-specific CFA adjustment of -160, to further
improve the use of signed 8-bit SFrame offsets.  This is similar to the
DWARF data alignment factor getting factored out from certain offsets
stored in DWARF CFI.

[1]: s390x ELF ABI, sections "Register Roles" and "Stack Frame
     Allocation", https://github.com/IBM/s390x-abi/releases
[2]: s390x ELF ABI, commit 4e38ad9c8a88 ("Document the CFA"),
     https://github.com/IBM/s390x-abi/commit/4e38ad9c8a88

include/
	* sframe.h (SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR): Define
	s390x-specific CFA offset alignment factor.
	(SFRAME_V2_S390X_CFA_OFFSET_ENCODE,
	SFRAME_V2_S390X_CFA_OFFSET_DECODE): Scale down/up by
	SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR.

libsframe/
	* doc/sframe-spec.texi (s390x,
	SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR): Document s390x-
	specific CFA offset alignment factor.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>