gas: section name substitution sequence

This patch adds the ability to automatically construct a section name
based on the prior section.

When gas is invoked with --sectname-subst, the occurrence of %S in a
section name will be substituted by the name of the current section. For
example:

	.macro exception_code
	.pushsection %S.exception
	[exception code here]
	.popsection
	.endm

	.text
	[code]
	exception_code
	[...]

	.section .init
	[init code]
	exception_code
	[...]

The first and second exception_code invocations create the
.text.exception and the .init.exception sections respectively.  This is
useful e.g. to discriminate between anciliary sections that are tied to
.init code and can be discarded at run time when initialization is over
vs anciliary sections tied to .text sections that need to stay resident.

	* as.c (show_usage): Document --sectname-subst.
	(parse_args): Add --sectname-subst.
	* as.h (flag_sectname_subst): New.
	* config/obj-elf.c (obj_elf_section_name): Add %S substitution.
	* doc/as.texinfo: Document it.

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