linux-user: Remove THREAD macro

linux-user: Remove THREAD macro

Back when we used to support compiling either with or without
NPTL threading library support, we used a macro THREAD which would
expand either to nothing (no thread support) or to __thread (threads
supported). For a long time now we have required thread support,
so remove the macro and just use __thread directly as other parts
of QEMU do.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Laurent Vivier <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-Id: <20180213132246[email protected]>
Signed-off-by: Laurent Vivier <[email protected]>