cmd/link, cmd/go: make version info easier to extract
Reading the version information to date has required evaluating
two pointers to strings (which themselves contain pointers to data),
which means applying relocations, which can be very system-dependent.
To simplify the lookup, inline the string data into the build info blob.
This makes go version work on binaries built with external linking
on darwin/arm64.
Also test that at least the very basics work on a trivial binary,
even in short mode.