runtime: `sysMemStat overflow` on linux-riscv64-unmatched

[bcmills]

#!watchflakes
post <- goarch == "riscv64" && log ~ `sysMemStat overflow`

(pulled out from #55167 (comment))

(attn @golang/riscv64; CC @golang/runtime)

See previously #52682.

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
post <- goarch == "riscv64" && log ~ `sysMemStat overflow`

2023-06-16 20:47 linux-riscv64-unmatched go@0183c1aa runtime.TestChan (log)

runtime: val=18446744073709518848 n=-40960
fatal error: sysMemStat overflow

runtime stack:
runtime.throw({0x4214ef?, 0x3f6c1c9223?})
	/tmp/workdir-host-linux-riscv64-unmatched/go/src/runtime/panic.go:1077 +0x44 fp=0x3f667fb650 sp=0x3f667fb628 pc=0x4cbc4
runtime.(*sysMemStat).add(0x74b4a0?, 0xffffffffffff6000)
	/tmp/workdir-host-linux-riscv64-unmatched/go/src/runtime/mstats.go:642 +0x88 fp=0x3f667fb670 sp=0x3f667fb650 pc=0x47350
runtime.(*mheap).allocSpan(0x74b4a0, 0x5, 0x0, 0x0?)
	/tmp/workdir-host-linux-riscv64-unmatched/go/src/runtime/mheap.go:1355 +0x490 fp=0x3f667fb710 sp=0x3f667fb670 pc=0x3cd68
...
	/tmp/workdir-host-linux-riscv64-unmatched/go/src/runtime/malloc.go:1346 +0x6c fp=0x3f40008bb0 sp=0x3f40008b90 pc=0x1f1bc
runtime.makeBucketArray(0x3a5160, 0x88?, 0x3a5160?)
	/tmp/workdir-host-linux-riscv64-unmatched/go/src/runtime/map.go:364 +0x190 fp=0x3f40008bd8 sp=0x3f40008bb0 pc=0x1fff0
runtime.hashGrow(0x3f400e3698?, 0x3f40008e90)
	/tmp/workdir-host-linux-riscv64-unmatched/go/src/runtime/map.go:1068 +0x98 fp=0x3f40008c10 sp=0x3f40008bd8 pc=0x21ca8
runtime.mapassign_fast64(0x3a5160, 0x3f40008e90, 0xa3)
	/tmp/workdir-host-linux-riscv64-unmatched/go/src/runtime/map_fast64.go:157 +0xf0 fp=0x3f40008c48 sp=0x3f40008c10 pc=0x23748
runtime_test.TestChan(0x3f698456c0)
	/tmp/workdir-host-linux-riscv64-unmatched/go/src/runtime/chan_test.go:173 +0xf64 fp=0x3f40008f78 sp=0x3f40008c48 pc=0x2b97f4
testing.tRunner(0x3f698456c0, 0x4451c8)

— watchflakes