Revert "Use eventfd in ChildProcess on Linux"
Revert "Use eventfd in ChildProcess on Linux"

This reverts commit b169f7b0d51fa9e7cf570479079369b9736093ff.

This caused `integer cast truncated bits` at
std/child_process.zig:801:12

Can be reproduced on my machine simply by running `make`.

zig-lang/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,zig-lang/zig,andrewrk/zig,raulgrell/zig,andrewrk/zig