git: Tweak .gitignore to ignore defined platform build directories.  - We should probably sink the platform build dirs into a common top-level dir.
git: Tweak .gitignore to ignore defined platform build directories.
 - We should probably sink the platform build dirs into a common top-level dir.

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@144662 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt