preadv2 is not supported on musl libc

[NobodyXu]

The preadv2 function is present on -gnu target, however when compiling on -musl target, which is surprising.

The jobserver-rs would like to use it to optimize it's performance rust-lang/jobserver-rs#90 (the try_acquire optimization has shipped once but reverted, so I opened another PR to use raw syscall directly).

FYI, my motivation is to that this API is used in cc-rs to improve performance.

[ptrca]

closed by #3762

[NobodyXu]

Thank you!