runtime: SIGSEGV in preemptone

[gopherbot]

#!watchflakes
default <- goarch == "riscv64" && builder == "linux-riscv64-mengzhuo" && `sigcode=1 addr=0xc0`  

Original flakes

#!watchflakes
default <- pkg == "golang.org/x/tools/internal/imports" && test == "TestModReplace2"

Issue created automatically to collect these failures.

Example (log):

=== RUN   TestModReplace2
SIGSEGV: segmentation violation
PC=0x54520 m=12 sigcode=1 addr=0xc0

goroutine 0 gp=0x3f504421c0 m=12 mp=0x3f50526708 [idle]:
runtime.preemptone(0x3f504421c0?)
	/home/swarming/.swarming/w/ir/x/w/goroot/src/runtime/proc.go:6297 +0x38 fp=0x3f5043ff28 sp=0x3f5043ff10 pc=0x54520
runtime.preemptall()
	/home/swarming/.swarming/w/ir/x/w/goroot/src/runtime/proc.go:6275 +0x60 fp=0x3f5043ff50 sp=0x3f5043ff28 pc=0x544c0
runtime.forEachPInternal(0x2fa878)
...
a3  0x223abf02	a4  0x3f98bb8000
a5  0x31a7d99	a6  0x29ab75fd
a7  0x1187f0	s2  0x3f5043fed0
s3  0x3f50526708	s4  0x3f50474000
s5  0x3f50241500	s6  0xffffffff
s7  0x4	s8  0x3f50038688
s9  0x3f5043fdc8	s10 0x2fa878
s11 0x3f504421c0	t3  0x2eb2a46908caf
t4  0xffffffffffffffff	t5  0x1913e15049b3
t6  0x3f50038408	pc  0x54520

— watchflakes