gas: add new command line option --scfi[=all,none]

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

The option --scfi=all ignores any existing .cfi_* directives already
contained in the provided assembler file.

To use SCFI, a target will need to:
    - define TARGET_USE_SCFI and TARGET_USE_GINSN, and other necessary
    definitions,
    - provide implementation of some APIs to help GAS understand the
    target specific semantics.

The --scfi=[all,none] may see more options added in future.  For
example, --scfi=inline, for dealing with inline asm may be added in the
future.  In this option, the GNU assembler will consume (and not ignore)
the compiler generated CFI for the code surrounding the inline asm.

Also document the option.

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