Fix subzero build for mac

Fix subzero build for mac

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

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-subzero.git;a=commit;h=44712d1

[Derek Schuff, 2014-06-17 18:42:42]

A little ugly with all the .data() everywhere but this might be preferable to
bringing in SmallString just for this.

[Jim Stichnoth, 2014-06-17 21:07:11]

LGTM, unless JF has any better ideas (e.g., OwningPtr).

[Derek Schuff, 2014-06-17 21:34:40]

Committed patchset #2 manually as r44712d1 (presubmit successful).