gas: dw2gencfi: minor rejig for cfi_sections_set and all_cfi_sections

- cfi_sections_set is best set to true in cfi_dot_startproc ().  Setting
  it to true again in other APIs (dot_cfi_endproc, dot_cfi_fde_data, and
  cfi_finish) is unnecessary.
- move setting the global var all_cfi_sections into cfi_set_sections ().

ChangeLog:
        * gas/dw2gencfi.c (cfi_set_sections): Set cfi_sections_set and
	cfi_sections here.
        (dot_cfi_startproc): Remove unnecessarily setting
	cfi_set_sections to true.
        (dot_cfi_endproc): Likewise.
        (dot_cfi_fde_data): Likewise.
        (cfi_finish): Likewise.