dwarf: Add -gdwarf{32,64} options

dwarf: Add -gdwarf{32,64} options

The following patch makes the choice between 32-bit and 64-bit DWARF formats
selectable by command line switch, rather than being hardcoded through
DWARF_OFFSET_SIZE macro.

The options themselves don't turn on debug info themselves, so one needs
to use -g -gdwarf64 or similar.

2020-12-04  Jakub Jelinek  <jakub@redhat.com>

	* common.opt (-gdwarf32, -gdwarf64): New options.
	* config/rs6000/rs6000.c (rs6000_option_override_internal): Default
	dwarf_offset_size to 8 if not overridden from the command line.
	* dwarf2out.c: Change all occurrences of DWARF_OFFSET_SIZE to
	dwarf_offset_size.
	* doc/invoke.texi (-gdwarf32, -gdwarf64): Document.