Introduce llgo.GoVersion() and use it to determine the version of Go we target

The version of Go that we target is not necessarily the same as
the version that we were compiled with. Also, the output of "$GOC --version"
is used by libgo to determine the value of runtime.Version(), and thus is
unstable during bootstrapping.
Introduce support for -print-multi-os-directory

Also, factor --version into the same code path.
Introduce support for inferring import and library paths from the installation prefix
Add scripts and Makefile for bootstrapping and installing llgo

Also update the README.md file.