Build RS runtime libraries using our arm and aarch64 triples only.

Bug: 17333374

Without this change, we might end up using different (broken) calling
conventions on non-ARM architectures. This ensures that all C source
files get built the same. Any .ll files can still use the appropriate
target arch-specific features.

Change-Id: I2658c49620d297cb1ef8b96b5b5d1597f5fd7361
2 files changed