Add support for riscv64 on Linux

[Civil]

Linux on riscv64 doesn't have support for Dup2 syscall,
but as in case with arm64 it defines Dup3 syscall.

• Rename syscall_dup_arm64 to syscall_dup3
• Add a conditional compilation comment there to compile it on riscv64 as well

Fixes #74

[codecov]

Codecov Report

Merging #75 (e36527e) into master (bf83969) will decrease coverage by 1.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
- Coverage   15.89%   14.56%   -1.33%     
==========================================
  Files          10        8       -2     
  Lines         258      206      -52     
==========================================
- Hits           41       30      -11     
+ Misses        212      171      -41     
  Partials        5        5              

Impacted Files	Coverage Δ
lock_file.go	58.97% <0.00%> (-5.61%)	⬇️
lock_file_unix.go	75.00% <0.00%> (-5.00%)	⬇️
signal.go	7.14% <0.00%> (-2.39%)	⬇️
daemon.go	0.00% <0.00%> (ø)
command.go	0.00% <0.00%> (ø)
daemon_unix.go	0.00% <0.00%> (ø)
syscall_dup_pre17.go	0.00% <0.00%> (ø)
os_executable_pre18.go	0.00% <0.00%> (ø)
os_executable.go

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