Introduce run_on_main_thread

Introduce run_on_main_thread

This introduces a way for a callback to be run on the main thread.

gdb/ChangeLog
2019-11-26  Tom Tromey  <tom@tromey.com>

	* run-on-main-thread.c: New file.
	* run-on-main-thread.h: New file.
	* unittests/main-thread-selftests.c: New file.
	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	main-thread-selftests.c.
	(HFILES_NO_SRCDIR): Add run-on-main-thread.h.
	(COMMON_SFILES): Add run-on-main-thread.c.

Change-Id: I16ef82f0564e9f8a524bdc64cb31df79a988ad9f