Auto merge of #53725 - tbu-:pr_getrandom_syscalls, r=alexcrichton
Auto merge of #53725 - tbu-:pr_getrandom_syscalls, r=alexcrichton

Reduce number of syscalls in `rand`

This skips the initial zero-length `getrandom` call and
directly hands the user buffer to the operating system, saving one
`getrandom` syscall.

aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust