Introduce procedure use_gdb_stub

Introduce procedure use_gdb_stub

This patch introduces the use_gdb_stub procedure, which allows getting
the right value of the use_gdb_stub variable/property in any all
situations.

When calling it before the $use_gdb_stub global variable has been set,
it will return the value of the use_gdb_stub property from the board
file.  This happens when tests want to bail out early (even before gdb
has been started) when the current test setup is a stub.

Otherwise, it returns the value of the $use_gdb_stub global.

It's possible for these two to differ when a test file overrides the
value of the global.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (use_gdb_stub): New procedure.