Small fixes to compile on aarch64

• fix x86_64 specific configuration bits
• remove invalid constexpr in aarch64 (neon) specific code
• small change to error messages (somehow fmtlib could not format YAML::Node)

Note: the test build was achieved following the instructions at https://codimd.web.cern.ch/s/G2qnvDjev

/cc @bcouturi @valassi