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 <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20180213132246.26844-1-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>