internal/buildcfg: initialize GOROOT to runtime.GOROOT
In the beginning the Go compiler was in C, and C had a function
'getgoroot' that returned GOROOT from either the environment or a
generated constant. 'getgoroot' was mechanically converted to Go
(as obj.Getgoroot) in CL 3046.
obj.Getgoroot begat obj.GOROOT. obj.GOROOT begat objabi.GOROOT,
which begat buildcfg.GOROOT.
As far as I can tell, today's buildcfg.GOROOT is functionally
identical to runtime.GOROOT(). Let's reduce some complexity by
defining it in those terms.
While we're thinking about buildcfg.GOROOT, also check whether it is
non-empty: if the toolchain is built with -trimpath, the value of
GOROOT might not be valid or meaningful if the user invokes
cmd/compile or cmd/link directly, or via a build tool other than
cmd/go that doesn't care as much about GOROOT. (As of CL 390024,
runtime.GOROOT will return the empty string instead of a bogus one
when built with -trimpath.)