compiler has regressions on mips64, ok'd by @nmeum for 3.12

We don't use grsec anymore.

Only cgo_test is disabled as `../misc/cgo/test` seems to hang for some
reason. All other tests pass on all architecture as we don't pass
-builmode=pie unconditionally anymore.

With default-buildmode-pie.patch removed we must pass this explicitly
since make.bash unsets GOFLAGS.

Instead set -buildmode=pie using the GOFLAGS environment variable on the
builders (see last commit). Users either have to pass -buildmode=pie
explicitly or configure their shell environment accordingly.

This fixes the `cannot find runtime/cgo`-warning and will allow running
more test cases of the go compiler test suite in the future.