Refine size constraints applied to win32pstatus ELF notes

Don't reject any win32pstatus notes smaller than minimum size for a
NOTE_INFO_THREAD.

This only happens to work because the Cygwin dumper tool currently
writes all these notes as the largest size of the union, (which wastes
lots of space in the core dump).

Instead, apply the appropriate size constraint for each win32pstatus
note type.

bfd/ChangeLog:

2020-07-11  Jon Turney  <jon.turney@dronecode.org.uk>

	* elf.c (elfcore_grok_win32pstatus): Don't apply size constraint
	for NOTE_INFO_THREAD to all win32pstatus ELF notes, instead apply
	appropriate size constraint for each win32pstatus note type.