fast_math: SIMD function library that can select to use NEON at runtime.

Only contains one function so far: fast_matrix_mul_4x4. C, SSE, and NEON impls.
Have fast_matrix_mul_4x4_sse accept unaligned matrices
Fix build for x86 android etc.
Merge pull request #202 from hrydgard/fast_math

Fast math mini-library