Add option to build a certain target (and -h option)

This should allow us to run make/ninja on a given target, e.g.
`llvm-build llc` will build only the llc target
`llvm-build check-llvm-codegen-aarch64` will only run a subset of the tests

This also adds a -h option.

Change-Id: I4b2d195ed7f7f4b23f84438068dd1e13a094e947
1 file changed