Default to --enable-compressed-debug-sections=gas for Linux/x86

--enable-compressed-debug-sections=gas added to binutils 2.26.  Make it
default for Linux/x86 targets in 2.27.

	* NEWS: Mention --enable-compressed-debug-sections=gas is the
	default for Linux/x86 targets.
	* configure.tgt (ac_default_compressed_debug_sections): Default
	to yes for Linux/x86 targets.