gn build: Make a couple of improvements to the unix toolchain.
gn build: Make a couple of improvements to the unix toolchain.

Add an asm tool (will be required for building sanitizer_common on
x64) and set a soname for DSOs so that anything that links against
them gets the correct DT_NEEDED.

Differential Revision: https://reviews.llvm.org/D56705

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@351167 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm