internal/goexperiment: add GOEXPERIMENT=boringcrypto

Not hooked up to everything else yet.

Copy of CL 395880, for setting up GOEXPERIMENT=boringcrypto
builder ahead of merge.

For #51940.

Change-Id: If842761f77d07329d88748990b95f4b39c2f153a
Reviewed-on: https://go-review.googlesource.com/c/go/+/397895
Trust: Russ Cox <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: rsc

src/internal/goexperiment/exp_boringcrypto_off.go

@@ -58,6 +58,7 @@ type Flags struct {
 	FieldTrack        bool
 	PreemptibleLoops  bool
 	StaticLockRanking bool
+	BoringCrypto      bool
 
 	// Unified enables the compiler's unified IR construction
 	// experiment.