Fix x86_64 mingw build

Fix x86_64 mingw build

PR build/27807 points out that my recent changes to the Windows port
missed a spot in win32-i386-low.cc -- a call to
win32_Wow64GetThreadContext remained, causing link errors in
gdbserver.  This happened because I tested an i686 build, but this
code is only used on an x86_64 build.

This patch fixes the bug.  I am checking it in.

gdbserver/ChangeLog
2021-05-03  Tom Tromey  <tromey@adacore.com>

	PR build/27807:
	* win32-i386-low.cc (i386_get_thread_context): Call
	Wow64GetThreadContext, not win32_Wow64GetThreadContext.