runtime: set gp.syscallbp from entersyscallblock_handoff

This was an oversight and is causing a few failures, most notably on
Solaris and Illumos, but also occasionally on the Linux builders.

Change-Id: I38bd28537ad01d955675f61f9b1d42b9ecdd1ef0
Reviewed-on: https://go-review.googlesource.com/c/go/+/580875
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index 54408db..1468c7c 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -4428,6 +4428,7 @@
 	save(pc, sp, bp)
 	gp.syscallsp = gp.sched.sp
 	gp.syscallpc = gp.sched.pc
+	gp.syscallbp = gp.sched.bp
 	if gp.syscallsp < gp.stack.lo || gp.stack.hi < gp.syscallsp {
 		sp1 := sp
 		sp2 := gp.sched.sp