cmd/llgo-dist: clean up ztypes_pnacl generated code

- don't leave obj_ dir in cwd.
- add pnacl build tag to generated file
Generate \uFFFD for invalid Unicode code points

Also, handle []rune(string) constants.

These changes enable the successful execution
of go/test/string_lit.go.
cmd/llgo-build: Add -emit-llvm flag

Useful for debugging linked IR.