[patch gas]: Add missing parts for pdata/xdata generation

[Kai Tietz <ktietz70 at googlemail dot com>]

Hello,

as preparation for the upcoming x64 SEH unwind-information patch for
gcc, this patch updates some missing parts of my intial support. It
writes now content into the unwind-information data and improves
parser. Also add some more comments to functions (all but complete,
but at least a start).  The testcases for x64 I'll provide when x64
feature for x64 unwind-information gets into gcc. So a qualified test
will be possible.

ChangeLog

        * config/obj-coff-seh.c
        (seh_getelm_data_size): New.
        (seh_read_offset): Handle negative values.
        (obj_coff_seh_push): Handle offset for save-register store.
        (obj_coff_seh_setframe): Add unwind-information for frame.
        (seh_store_elm_data): New.
        (seh_getelm_data_size): Return additionally unaligned element count.
        (seh_make_unwind_entry): Correct tweak about element count.

Tested for x86_64-pc-mingw32. (Build test for i686-pc-cygwin and
i686-pc-mingw32). Ok for apply?

Regards,
Kai

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