Auto merge of #2664 - rschulman:master, r=joshtriplett
Auto merge of #2664 - rschulman:master, r=joshtriplett

Add 400-series syscalls to musl riscv64 definitions

This PR adds 19 missing syscalls to the riscv64 musl definitions. The missing calls were copied directly from the riscv64 GNU definitions. The missing definitions (in particular `SYS_clone3`) are preventing compiling `std` for `riscv64gc-unknown-linux-musl`.

rust-lang/libc,rust-lang/libc,rust-lang/libc