libiberty: Add BigObj COFF support for LTO on Windows targets [PR122472]

This patch adds support for the BigObj COFF object file format to libiberty's
simple-object-coff.c. BigObj extends regular COFF to support a 32-bit section
count.

BigObj differs from COFF in a few ways:

* A different header structure
* 32-bit section counts instead of 16-bit
* 32-bit symbol section numbers instead of 16-bit
* 20-byte symbols instead of 18-byte symbols
  (due to the extended section numbers)

For a more detailed summary, read my blog post on this subject:
https://peter0x44.github.io/posts/bigobj_format_explained/

libiberty/ChangeLog:

	PR target/122472
	* simple-object-coff.c (struct external_filehdr_bigobj): New
	structure for BigObj file header.
	(bigobj_magic): New constant for BigObj magic bytes.
	(struct external_syment_bigobj): New structure for BigObj
	20-byte symbol table entries.
	(union external_auxent_bigobj): New union for BigObj 20-byte
	auxiliary symbol entries.
	(struct simple_object_coff_read): Add is_bigobj flag and make
	nscns 32-bit to support both formats.
	(struct simple_object_coff_attributes): Add is_bigobj flag.
	(simple_object_coff_match): Add BigObj format detection.
	(simple_object_coff_read_strtab): Use format-specific symbol
	size when calculating string table offset.
	(simple_object_coff_attributes_merge): Check is_bigobj flag.
	(simple_object_coff_write_filehdr_bigobj): New function.
	(simple_object_coff_write_to_file): Add logic for writing
	BigObj vs regular COFF format with appropriate symbol
	and auxiliary entry structures.

Signed-off-by: Peter Damianov <peter0x44@disroot.org>
Signed-off-by: Jonathan Yong <10walls@gmail.com>

Author: Peter0x44