LD: Support fixed-size sections some psABIs may require

LD: Support fixed-size sections some psABIs may require

Define a SEC_FIXED_SIZE section flag for target backends to use for
output sections whose size has been fixed in the psABI.  The size of
such sections will not be changed anyhow by the generic linker and it is
up to the target backend to get their size right.

	bfd/
	* section.c (SEC_FIXED_SIZE): New macro.
	* bfd-in2.h: Regenerate.

	ld/
	* ldlang.c (insert_pad): Do not change output section's size if
	SEC_FIXED_SIZE is set in the flags.
	(size_input_section): Likewise.
	(lang_size_sections_1): Likewise.
	(lang_reset_memory_regions): Likewise.