Rollup merge of #137349 - thaliaarchi:io-optional-methods/zkvm, r=Noratrieb

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 risc0/risc0#2853.

cc `@bobbobbio,` `@flaub`

r? `@Noratrieb`

Tracked in rust-lang/rust#136756

Author: tgross35