x/build: add a builder for GOEXPERIMENT=runtimefreegc

[thepudds]

@mknyszek suggested it likely could be worthwhile to add a builder that enables GOEXPERIMENT=runtimefreegc in order to help with coverage.

GOEXPERIMENT=runtimefreegc is a disabled-by-default GOEXPERIMENT in Go 1.26. It currently includes changes in the runtime and compiler.

(For an overview, see the "Introduction" section in https://go.dev/design/74299-runtime-freegc, or see https://go.dev/cl/710015 for some sample performance results.)

Filing this issue for discussion and tracking.

Thanks!

CC @dmitshur, @golang/release

[thepudds]

(Also, especially with the quiet week about to start and the holiday season upon us, I should add that I’m in no particular rush. I could for example ping this issue in early January if that helps.)