The subzero mac build fails with errors like the following:
/Users/dschuff/code/nacl/native_client/toolchain_build/src/subzero/src/IceGlobalContext.cpp:116: error: ISO C++ forbids variable-size array 'NameBase'

Replace the variable-length array with llvm::SmallVector which will still
allow stack allocation most of the time.

R=stichnot@chromium.org
BUG=build subzero on the bots

Review URL: https://codereview.chromium.org/335343005