unix: replace readlink in mkall.sh with portable shell commands
unix: replace readlink in mkall.sh with portable shell commands

Fixes golang/go#35997

Change-Id: I1c489a126646a0ff4abf9e8b7ba357ff62cb66ac
GitHub-Last-Rev: e01b1c0e8baa28713e7d2235f2f55086a97d709c
GitHub-Pull-Request: golang/sys#49
Reviewed-on: https://go-review.googlesource.com/c/sys/+/210157
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/sys,multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,golang/sys,multiplay/golang-sys