Don't hardcode CONTEXT size for a NOTE_INFO_THREAD win32pstatus note

Don't hardcode the size of the Win32 API thread CONTEXT type read from a
NOTE_INFO_THREAD win32pstatus note (since it's different on different
architectures).

bfd/ChangeLog:

2020-07-01  Jon Turney  <jon.turney@dronecode.org.uk>

	* elf.c (elfcore_grok_win32pstatus): Don't hardcode the size of
	the Win32 API thread CONTEXT type read from a NOTE_INFO_THREAD
	win32pstatus note.