Implement `read_buf` for zkVM stdin
For the zkVM, even when a guest buffer is uninitialized, from the host's perspective it is just a normal piece of memory which was initialized before letting the guest write into it. This makes `sys_read` safe to use with an uninitialized buffer. See https://github.com/risc0/risc0/issues/2853.
cc `@bobbobbio,` `@flaub`
r? `@Noratrieb`
Tracked in https://github.com/rust-lang/rust/issues/136756
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