Enable async mode on supported targets in target_resume.

Enabling async mode above the target layer removes duplicate code in
::resume methods of async-capable targets.  Commit 5b6d1e4f
("Multi-target support") enabled async mode in do_target_resume after
target_resume returns which is a step in this direction.  However,
other callers of target_resume such as target_continue do not enable
async mode.  Rather than enabling async mode in each of the callers
after target_resume returns, enable async mode at the end of
target_resume.