internal/runtime: fix assembly for spectre retpoline instrumentation
In the last year we added two CALLs whose targets are loaded from
memory. Change them to call from a register so that the instrumentation
for spectre mitigations works.
This change also adds a smoke test for the spectre build flags.
For #77420.
Change-Id: I35ec723449ff6a712bcce3276bf1df3fa932bddc
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/741541
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Pratt <mpratt@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>