Always default to DWARF2 debug for cygwin and mingw

Always default to DWARF2 debug for cygwin and mingw

This removes the fallback to STABS as default for cygwin and mingw
when the assembler does not support .secrel32 and the default is
to emit 32bit code.  Support for .secrel32 was added to binutils 2.16
released in 2005 so instead document that as requirement.

I left the now unused check for .secrel32 in configure around
in case somebody wants to turn that into an error or warning.

2021-09-10  Richard Biener  <rguenther@suse.de>

	* config/i386/cygming.h: Always default to DWARF2 debugging.
	Do not define DBX_DEBUGGING_INFO, that's done via dbxcoff.h
	already.
	* doc/install.texi: Document binutils 2.16 as minimum
	requirement for mingw.