src/runtime: add runtime.Version()

This adds the `Version()` function of the `runtime` package which embeds
the go version that was used to build tinygo.

For programs that are compiled with tinygo the version can be overriden
via the:
`tinygo build -ldflags="-X 'runtime.buildVersion=abc'"` flag.
Otherwise it will continue to use the go version with which tinygo was
compiled.

src/runtime/extern.go

@@ -3,3 +3,15 @@ package runtime
 func Callers(skip int, pc []uintptr) int {
 	return 0
 }
+
+// buildVersion is the Tinygo tree's version string at build time.
+//
+// This is set by the linker.
+var buildVersion string
+
+// Version returns the Tinygo tree's version string.
+// It is the same as goenv.Version, or in case of a development build,
+// it will be the concatenation of goenv.Version and the git commit hash.
+func Version() string {
+	return buildVersion
+}