runtime: port of builtins to ABIInternal breaks windows-amd64-regabi builder

[mknyszek]

Failing as of https://golang.org/cl/308931 with:

exit status 0xc0000005

It appears to fail consistently in net/http/internal.

CC @aclements @cherrymui

[aclements]

Failure is in net/http/pprof, actually.

[aclements]

Looks like runtime·externalthreadhandler (in assembly) calls runtime·memclrNoHeapPointers with some very specific assumptions about what it's going to smash or not smash.

[gopherbot]

Change https://golang.org/cl/309633 mentions this issue: runtime: use newm for profileloop

[gopherbot]

Change https://golang.org/cl/309634 mentions this issue: runtime: eliminate externalthreadhandler

[gopherbot]

Change https://golang.org/cl/309632 mentions this issue: runtime: use compileCallback for ctrlhandler