build: implement GOEXPERIMENT again in runtime, and add to liblink

For Austin's framepointer experiment.

Change-Id: I81b6f414943b3578924f853300b9193684f79bf4
Reviewed-on: https://go-review.googlesource.com/2994
Reviewed-by: Austin Clements <austin@google.com>
diff --git a/src/liblink/sym.c b/src/liblink/sym.c
index e068371..35b88f4 100644
--- a/src/liblink/sym.c
+++ b/src/liblink/sym.c
@@ -90,6 +90,7 @@
 	char *p;
 	char buf[1024];
 
+	linksetexp();
 	nuxiinit(arch);
 	
 	ctxt = emallocz(sizeof *ctxt);