gas: sframe: guard block with support_sframe_p

gas: sframe: guard block with support_sframe_p

SFrame is generated for ELF targets only.  Guard the block with
support_sframe_p like others for consistency.

Also, in a later commit, we would like to do a:
  elf_section_type (sframe_seg) = SHT_GNU_SFRAME;

This cannot be done for non-ELF targets, hence the need to guard with
some pre-processor conditional to enable it for only OBJ_ELF.  Guarding
with support_sframe_p works for now, because those targets that support
SFrame define support_sframe_p:
  - x86_64 and aarch64 define support_sframe_p when OBJ_ELF is defined
  - s390x has no non-LEF target.

We continue to issue an error on targets where SFrame is not supported:
    .sframe not supported for target

gas/
	* dw2gencfi.c (cfi_finish): Guard with support_sframe_p.
	(support_sframe_p): Remove stub to define to false for backends
	not supporting SFrame.