runtime/internal/sys: define Goexperiment

The gc toolchain defines Goexperiment based on the environment
variable GOEXPERIMENT when the toolchain is built.  We just always set
Goexperiment to the empty string.

Change-Id: I1bceb5d64af63dd75f2878eb76f6903b8ac1152a
Reviewed-on: https://go-review.googlesource.com/41292
Reviewed-by: Than McIntosh <thanm@google.com>
2 files changed