runtime,cmd/link: "found pointer to free object" during asyncPreempt2 on ios-arm64-corellium

[bcmills]

##### Test execution environment.
# GOARCH: arm64
# CPU: 
# GOOS: ios
# OS Version: Darwin 20.6.0 Darwin Kernel Version 20.6.0: Tue Aug 24 21:10:43 PDT 2021; root:xnu-7195.140.44~1/RELEASE_ARM64_T8010 iPhone9,1

…
# crypto/sha256.test
runtime: marked free object in span 0x100ebd108, elemsize=24 freeindex=82 (bad use of unsafe.Pointer? try -d=checkptr)
…
fatal error: found pointer to free object
…
goroutine 1 [runnable]:
runtime.asyncPreempt2()
	/tmp/workdir-host-ios-arm64-corellium-ios/go/src/runtime/preempt.go:308 +0x40 fp=0x1300e1c10 sp=0x1300e1bf0 pc=0x1008488e0
runtime.asyncPreempt()
	/tmp/workdir-host-ios-arm64-corellium-ios/go/src/runtime/preempt_arm64.s:47 +0xa0 fp=0x1300e1e00 sp=0x1300e1c10 pc=0x100877980
crypto/sha256.blockGeneric(0x1300e1f98?, {0x1044a8000?, 0x1300cb320?, 0x1044a8000?})
	/tmp/workdir-host-ios-arm64-corellium-ios/go/src/crypto/sha256/sha256block.go:90 +0x1bc fp=0x1300e1f30 sp=0x1300e1e10 pc=0x100939f3c
crypto/sha256.block(0x1300e1fe8?, {0x1044a8000?, 0x1300c83a8?, 0x1044a8000?})
	/tmp/workdir-host-ios-arm64-corellium-ios/go/src/crypto/sha256/sha256block_arm64.go:16 +0x7c fp=0x1300e1f90 sp=0x1300e1f30 pc=0x10093a0dc
crypto/sha256.(*digest).Write(0x1300e2110, {0x1044a8000?, 0x1000, 0x1000?})
	/tmp/workdir-host-ios-arm64-corellium-ios/go/src/crypto/sha256/sha256.go:198 +0x13c fp=0x1300e1ff0 sp=0x1300e1f90 pc=0x10093993c
cmd/internal/codesign.Sign({0x104496000?, 0x104493910?, 0x2?}, {0x100ad9198, 0x1300c83a8}, {0x100a0c666, 0x5}, 0x235d10, 0x1300e2268?, 0x10085f7c0?, ...)
	/tmp/workdir-host-ios-arm64-corellium-ios/go/src/cmd/internal/codesign/codesign.go:264 +0x400 fp=0x1300e21b0 sp=0x1300e1ff0 pc=0x10093abe0
cmd/link/internal/ld.machoCodeSign(0x130120000, {0x16f5ef1e8, 0x51})
	/tmp/workdir-host-ios-arm64-corellium-ios/go/src/cmd/link/internal/ld/macho.go:1558 +0x36c fp=0x1300e2320 sp=0x1300e21b0 pc=0x10099fe2c
cmd/link/internal/ld.(*Link).hostlink(0x130120000)
	/tmp/workdir-host-ios-arm64-corellium-ios/go/src/cmd/link/internal/ld/lib.go:1799 +0x4b18 fp=0x1300e2c80 sp=0x1300e2320 pc=0x1009913d8
cmd/link/internal/ld.Main(_, {0x10, 0x20, 0x1, 0x1f, 0x1e, 0x7c00000, {0x100a14461, 0x14}, {0x100a17ba8, ...}, ...})
	/tmp/workdir-host-ios-arm64-corellium-ios/go/src/cmd/link/internal/ld/main.go:378 +0x1448 fp=0x1300e2f10 sp=0x1300e2c80 pc=0x1009a3d88
main.main()
	/tmp/workdir-host-ios-arm64-corellium-ios/go/src/cmd/link/main.go:69 +0xc5c fp=0x1300e3f70 sp=0x1300e2f10 pc=0x100a0ac3c
runtime.main()
	/tmp/workdir-host-ios-arm64-corellium-ios/go/src/runtime/proc.go:250 +0x250 fp=0x1300e3fd0 sp=0x1300e3f70 pc=0x100849de0
runtime.goexit()
	/tmp/workdir-host-ios-arm64-corellium-ios/go/src/runtime/asm_arm64.s:1155 +0x4 fp=0x1300e3fd0 sp=0x1300e3fd0 pc=0x1008770b4
…
FAIL	crypto/sha256 [build failed]

(CC @golang/runtime @golang/ios)