Fix building on Amazon Linux 2

[JulianGro]

This PR allows using vircadia-builder to build on Amazon Linux 2 (again?).

• Use "cmake3" on Amazon Linux 2 as "cmake" still only provides version 2.
• Remove patchelf as dependency as there is no matching package in the Amazon Linux 2 repositories.
• Do not patch paths with patchelf on Amazon Linux 2 as it is not available.
• Add info about perl-Term-ReadLine-Gnu as there is no matching package in the Amazon Linux 2 repositores.
• Skip user daemon reload as Amazon Linux 2 does not support user services.