Make assignments to dot keep an empty output section.

An assignment to dot in an output section that allocates space of
course keeps the output section.  Here, I'm changing the behaviour for
assignments that don't allocate space.  The idea is not so much to
allow people to force output of an empty section with ". = .", but
to fix cases where an otherwise empty section has padding added by an
alignment expression that changes with relaxation or .eh_frame
editing.  Such a section might have zero size before relaxation and so
be stripped incorrectly.

ld/
	* ld.texinfo (Output Section Discarding): Mention assigning to dot
	as a way of keeping otherwise empty sections.
	* ldexp.c (is_dot, is_value, is_sym_value, is_dot_ne_0,
	is_dot_plus_0, is_align_conditional): New predicates.
	(exp_fold_tree_1): Set SEC_KEEP when assigning to dot inside an
	output section, except for some special cases.
	* scripttempl/elfmicroblaze.sc: Use canonical form to align at
	end of .heap and .stack.
ld/testsuite/
	* ld-shared/elf-offset.ld: Align end of .bss with canonical form
	of ALIGN that allows an empty .bss to be removed.
	* ld-arm/arm-dyn.ld: Likewise.
	* ld-arm/arm-lib.ld: Likewise.
	* ld-elfvsb/elf-offset.ld: Likewise.
	* ld-mips-elf/mips-dyn.ld: Likewise.
	* ld-mips-elf/mips-lib.ld: Likewise.
	* ld-arm/arm-no-rel-plt.ld: Remove duplicate ALIGN.
	* ld-powerpc/vle-multiseg-1.ld: Remove ALIGN at start of section.
	ALIGN address of section instead.
	* ld-powerpc/vle-multiseg-2.ld: Likewise.
	* ld-powerpc/vle-multiseg-3.ld: Likewise.
	* ld-powerpc/vle-multiseg-4.ld: Likewise.
	* ld-powerpc/vle-multiseg-6.ld: Likewise.
	* ld-scripts/empty-aligned.d: Check section headers not program
	headers.  Remove xfail and notarget.
	* ld-scripts/empty-aligned.t: Use canonical ALIGN for end of .text2.
Miscellaneous ld tidies

Localise a struct, prevent an unneeded symbol lookup, and fix a
testcase.

ld/
	* ld.h (struct map_symbol_def): Move to..
	* ldlang.h: ..here.
	* ldlang.c (print_assignment): Don't set expld.assign_name to dot.
ld/testsuite/
	* ld-scripts/pr14962-2.d: Correct target triple.