gas: add new command line option --scfi=experimental

When the command line option --scfi=experimenta is passed to the GNU
assembler, it will synthesize DWARF call frame information (CFI) for the
input assembly.

The option --scfi=experimental will also ignore most of the existing
.cfi_* directives, if already contained in the provided input file.
Only the following CFI directives will not be ignored:
  - .cfi_sections,
  - .cfi_label,
  - .cfi_signal_frame

To use SCFI, a target will need to:
    - define TARGET_USE_SCFI and TARGET_USE_GINSN, and other necessary
    definitions,
    - provide means to help GAS understand the target specific instruction
    semantics by creating ginsns.

The upcoming support for SCFI is inteded to be experimental, hence the
option --scfi=experimental.  The --scfi= may see more options like
--scfi=[all,none] added in future, once the SCFI support in GAS is
mature and robust.  The offering may also see for example, an
--scfi=inline option for dealing with inline asm may be added in the
future.  In --scfi=inline option, the GNU assembler may consume (and not
ignore) the compiler generated CFI for the code surrounding the inline
asm.

Also document the option.

gas/
        * as.c (show_usage): Add support for --scfi=experimental.
        (parse_args): Likewise.
        * as.h (enum synth_cfi_type): Define new type.
        * doc/as.texi: Document the new option.