Implement incstat in vasm

Summary: Added the addqim instruction to support incstat in vasm, eliminate the
incstat macro instruction.

Also remove the Stats::enabled() check in emitCall(); it was left over
from when we had trampolines. Further, delete the rip-addressing asserts;
one had a bug, and they were only useful before we changed RIPRelativeRef
to take an absolute address.

Remove the pushf/popf; we only emitIncStat() in places where its okay
to clobber flags. And if we make a mistake, vasm-xls will assert about
having two live status flags. And since its vasm, the fix can easily
use scratch registers and lea to avoid modifying flags. (load,lea,store).

Reviewed By: @jasone

Differential Revision: D1620331
Fix some request locals

Summary: icu's default locale can just be thread local.
There's no need to initialize s_signal_handlers on each request;
we can avoid the issues that solved by testing to see if its been
initialized. If not, the thread can't be expecting any signals.

Reviewed By: @bertmaher

Differential Revision: D1625632
Better handling of RequestEventHandlers

Summary: Its possible for a RequestEventHandler to get re-initialized
after its requestShutdown method has been called.

At the end of the request, we destroy the ExecutionContext, and so we
are left with a RequestEventHandler whose inited flag is set, but which
is not in the ExecutionContext. As a result, we never call requestInit
or requestShutdown for that handler ever again.

This tries to prevent the obvious ways that this can happen, and checks
for it when destroying the ExecutionContext.

Reviewed By: @ptarjan

Differential Revision: D1623445