FTBS on riscv64

[Vladimir Smirnov]

syscall_dup.go:11:9: undefined: syscall.Dup2

Problem is the same as with arm64 - new platforms do not have syscall.Dup2, but have syscall.Dup3 instead.

Proper solution would be to get a list of all platforms that have Dup2 syscall and whitelist them for old one and use new one for all other platforms (as new platforms will likely implement Dup3 syscall and not Dup2).

Or at least to:

1. Rename syscall_dup_arm64.go to not include arch name
2. Build it on:

// +build linux
// +build arm64 riscv64

3. Do not build old file on riscv64 (simple header modification).