Skip two more attach tests when testing against stub-like targets

Skip two more attach tests when testing against stub-like targets

This patch updates two attach tests to use utility procs for checking if
the attach test should run and for launching the program to be attached, as
follows:

1) Use can_spawn_for_attach instead of is_remote target
2) Use spawn_wait_for_attach instead of exec/sleep

Tested (1) with i686-mingw32 host and i686-pc-linux-gnu build/target and
both with x86_64 Ubuntu.

gdb/testsuite/ChangeLog:

	* gdb.base/attach-pie-noexec.exp: Use can_spawn_for_attach
	instead of checking whether the target board is remote and
	use spawn_wait_for_attach instead of exec/sleep.
	* gdb.base/attach-twice.exp: Likewise.