m68k: tag floating-point ABI used

m68k: tag floating-point ABI used

This patch adds GNU attribute support to m68k and utilises it to tag the
floating-point calling convention used (hard-float or soft-float). It enables
the linker to ensure linked objects use a consistent floating-point ABI and
allows tools like GDB to infer the ABI used from the ELF file. It is based on
similar work done for PowerPC.

bfd/
	* elf32-m68k.c (m68k_elf_merge_obj_attributes): New function.
	(elf32_m68k_merge_private_bfd_data): Merge GNU attributes.
binutils/
	* readelf.c (display_m68k_gnu_attribute): New function.
	(process_arch_specific): Call display_m68k_gnu_attribute for EM_68K.
gas/
	* config/tc-m68k.c (m68k_elf_gnu_attribute): New function.
	(md_pseudo_table): Handle "gnu_attribute".
	* doc/as.texi: Document GNU attribute for M68K.
include/
	* elf/m68k.h: Add enum for GNU object attribute with floating point
	tag name and values.
ld/
	* testsuite/ld-m68k/attr-gnu-4-0.s: New file.
	* testsuite/ld-m68k/attr-gnu-4-1.s: Likewise.
	* testsuite/ld-m68k/attr-gnu-4-2.s: Likewise.
	* testsuite/ld-m68k/attr-gnu-4-00.d: Likewise.
	* testsuite/ld-m68k/attr-gnu-4-01.d: Likewise.
	* testsuite/ld-m68k/attr-gnu-4-02.d: Likewise.
	* testsuite/ld-m68k/attr-gnu-4-10.d: Likewise.
	* testsuite/ld-m68k/attr-gnu-4-11.d: Likewise.
	* testsuite/ld-m68k/attr-gnu-4-12.d: Likewise.
	* testsuite/ld-m68k/attr-gnu-4-20.d: Likewise.
	* testsuite/ld-m68k/attr-gnu-4-21.d: Likewise.
	* testsuite/ld-m68k/attr-gnu-4-22.d: Likewise.
	* testsuite/ld-m68k/m68k.exp: Run the new tests.