thread.h: Remove trailing semicolons from Coverity qemu_mutex_lock() etc

All the Coverity-specific definitions of qemu_mutex_lock() and friends
have a trailing semicolon. This works fine almost everywhere because
of QEMU's mandatory-braces coding style and because most callsites are
simple, but target/s390x/sigp.c has a use of qemu_mutex_trylock() as
an if() statement, which makes the ';' a syntax error:
"../target/s390x/sigp.c", line 461: warning #18: expected a ")"
if (qemu_mutex_trylock(&qemu_sigp_mutex)) {
^

Remove the bogus semicolons from the macro definitions.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200319193323.2038-4-peter.maydell@linaro.org