cmd/go/internal/security: add -ftls-model to valid compiler flags
Allow -ftls-model to be passed in to a system compiler. It does not
allow arbitrary code execution. See
https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#index-ftls-model
for documentation for the -ftls-model flag.
Fixes #69711
Change-Id: I842a96832e6858e62c171401d13baa3391d6d00a
Reviewed-on: https://go-review.googlesource.com/c/go/+/617136
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ian Lance Taylor <iant@google.com>