Don't rely on gcc extensions

Summary:
Variable-length arrays of non-POD data types are a gcc extension
not currently supported by clang.

Change-Id: I9cd1aed742ac70e642771ff455582727fbafaec2
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>

Test Plan: verified

Reviewers: randall.rushing, jriordan001, anders3408

Reviewed By: randall.rushing

Differential Revision: http://review.codefirex.com/D284