Use gcc for arm64 target when clang fails.
Use gcc for arm64 target when clang fails.

a64/cpu-a64.cc does not compile with clang for arm64 target:
.../a64/cpu-a64.cc:69:39: error: value size does not match register size
specified by the constraint and modifier [-Werror,-Wasm-operand-widths]

BUG: 18734423
Change-Id: Ib56078cadeb279eda18b32070885edf43b85b00c

geekboxzone/mmallow_external_vixl,geekboxzone/mmallow_external_vixl,geekboxzone/mmallow_external_vixl