bthread: fix simd memory align issue #2178

Job	Run time
gcc-compile-with-make-protobuf	17m 38s
compile-with-make	16m 9s
gcc-compile-with-bazel	10m 39s
compile-with-cmake	17m 11s
clang-compile-with-make-protobuf	19m 55s
clang-compile-with-boringssl	16m 25s
gcc-compile-with-boringssl	11m 53s
clang-compile-with-bazel	14m 46s
clang-unittest	24m 42s
gcc-compile-with-bazel-all-options	11m 20s
clang-unittest-asan	24m 46s
clang-compile-with-bazel-all-options	15m 45s
	3h 21m 9s

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