We recently bumped our minimum required version of GCC to 7.4
and Clang to 6.0, and those compiler versions should support
the __thread keyword already.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211028185910.1729744-2-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>