Misc fixes for compiler_rt

1. Exclude lib/atomic-arm.c when building compiler_rt with clang.
   The file is to implement intrinsics generated by gcc
2. Exclude lib/atomic.c when building compiler_rt with gcc which
   has different sets of atomic intrinsics
3. Expand() logbl() as logb() because logbl doesn't exist in Android
   until API18, and "long double" is the same as "double"

Change-Id: I59921910a48f77e2e5f5da584a95cc64e6facf3f