cmd/llgo-dist: factor some code out to llgo/build

Chiefly, the GOOS/GOARCH detection code.
cmd/llgo-build: simple build tool

Factored out package building code from llgo-dist. llgo-dist
now builds this tool, with default triple, path to llgo, and
LLVM bin dir. llgo-dist then uses this tool to build the
standard library.
Merge branch 'master' of github.com:axw/llgo