Improve RS prebuilts script.

- Dynamically configure the number of cores used for parallel make.
- Make sure that we actually build LLVM from scratch.
- Fix build of host "bcc". We were building host and device "bcc" before,
  which was unnecessary. Now we just force the .so files to be built, which
  ensures a dependency on host "bcc".

Change-Id: I1d85f1f2c2405a4476275139705a5ae4faf29e00
1 file changed