Fix cross-arch build compilation error about missing O_RDONLY

[stephentoub]

I hit an error with the latest Visual Studio preview build.
cc: @janvorli

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Issue Details

---

cc: @janvorli

Author:	stephentoub
Assignees:	stephentoub
Labels:	area-Infrastructure-coreclr
Milestone:	-

[adamsitnik]

I confirm that it solves the problem:

[hoyosjs]

Odd one hit, looks like #65914

Assert failure(PID 566 [0x00000236], Thread: 590 [0x024e]): waitResult == WAIT_TIMEOUT
    File: /__w/1/s/src/coreclr/vm/tieredcompilation.cpp Line: 507
    Image: /root/helix/work/correlation/dotnet

For the reflection issue created #66100 as it's hitting several builds.

[janvorli]

LGTM, thank you!