fix(arm64): fix pacman build, pacman & rpm artifact names

[cwillisf]

Previous behavior: pacman files built for arm64 could not be installed. Instead, pacman would return errors similar to:

error: failed to prepare transaction (package architecture is not valid)
:: package yourpackage-arm64 does not have a valid architecture

New behavior: pacman files built for arm64 install correctly.

This change corrects the value returned by toLinuxArchString when arch is Arch.arm64 and targetName is pacman. Pacman calls this architecture aarch64 whereas other targets call it arm64. Compare to the existing code for ia32, which pacman calls i686 and others call i386.

This change also adjusts the value returned by getArtifactArchName to better match convention when arch is Arch.arm64: both pacman and rpm use aarch64 in package file names for this architecture. There may be more cases here which need to be corrected, but I'm currently only aware of these two.