Add a flag that reduces LLVM's dependency size

[MilesConn]

As per this Stack Overflow post it suggests adding the -DBUILD_SHARED_LIBS=on flag.

I have another commit that adds LLVM_TARGETS_TO_BUILD=WebAssembly but I haven't tested it yet so I'll do that.

Note that the LLVM docs indicate this flag is only recommended for LLVM developers so I'm not sure if this is a good idea or not. It does reduce build sizes from 80G to ~30G.

[arjunr2]

Reducing dependency size is good. As for targets to build, in addition to Wasm, I'd like to have the host platform as a target for clang too. If you can add that in, it would be great.