gas: Add --gdwarf-cie-version command line flag

gas: Add --gdwarf-cie-version command line flag

Add a flag to control the version of CIE that is generated.  By
default gas produces CIE version 1, and this continues to be the
default after this patch.

However, a user can now provide --gdwarf-cie-version=NUMBER to switch
to either version 3 or version 4 of CIE, version 2 was never released,
and so causes an error as does any number less than 1 or greater than
4.

Producing version 4 CIE requires two new fields to be added to the
CIE, an address size field, and an segment selector field.  For a flat
address space the DWARF specification indicates that the segment
selector should be 0, and the address size fields just contains the
address size in bytes.  For now we support 4 or 8 byte addresses, and
the segment selector is always produced as 0.  At some future time we
might need to allow targets to override this.

gas/ChangeLog:

	* as.c (parse_args): Parse --gdwarf-cie-version option.
	(flag_dwarf_cie_version): New variable.
	* as.h (flag_dwarf_cie_version): Declare.
	* dw2gencfi.c (output_cie): Switch from DW_CIE_VERSION to
	flag_dwarf_cie_version.
	* doc/as.texi (Overview): Document --gdwarf-cie-version.
	* NEWS: Likewise.
	* testsuite/gas/cfi/cfi.exp: Add new tests.
	* testsuite/gas/cfi/cie-version-0.d: New file.
	* testsuite/gas/cfi/cie-version-1.d: New file.
	* testsuite/gas/cfi/cie-version-2.d: New file.
	* testsuite/gas/cfi/cie-version-3.d: New file.
	* testsuite/gas/cfi/cie-version-4.d: New file.
	* testsuite/gas/cfi/cie-version.s: New file.

include/ChangeLog:

	* dwarf2.h (DW_CIE_VERSION): Delete.

Change-Id: I9de19461aeb8332b5a57bbfe802953d0725a7ae8