tweak source code to compile with clang

[charmoniumQ]

Here are the error messages I fixed:

error: struct 'static_bin_t' was previously declared as a class; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Werror,-Wmismatched-tags] solution: change "struct" -> "class" ../src/objsizes.cc:292:5 ../src/objsizes.cc:283:5

error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces] solution: adding braces is a no-brainer; doesn't change the meaning of code. ../src/large_malloc.cc:22:28 ../src/huge_malloc.cc:13:27 ../src/small_malloc.cc:7:70

error: unknown attribute 'optimize' ignored [-Werror,-Wunknown-attributes] solution: guard the GCC loop-unrolling directive, so that it is only enabled in GCC, and add a clang-guarded loop-unrolling directive ../src/cache.cc:358:16 ../src/cache.cc:370:16 ../src/cache.cc:679:16 ../src/cache.cc:694:16

error: unused variable 'unlock_spin_count' [-Werror,-Wunused-const-variable] solution: comment out ../src/futex_mutex.cc:35:18