Did some search and came to the conclusion that it is a combined effect from using clang 10.x with 4.26.x + old amd processor. Its not only your cpu all Phenom ii are affected.
Possible approaches
-
Hack: This Unreal engine will work using clang 6.2 to clang 10.1. So either hack the toolchain and use v16 clang-9.0.1
or
use make an empty text file at Engine/Build/PerforceBuild.txt . Then install clang 9.0.1 on system and try to install it. (Warning: there is no easy going back once system clang is used like with perforce hack!) -
File a bug report as epic can change these codes to make 4.26.x and above to work on older amd processors when using this version of clang. (that is if they still support these processors from amd, they have stated in system requirement that any 4+ core amd/intel are supported, so they are to be blamed).
(4.26.x is not good compared to 4.25.4 for production!)