[compiler-rt] Restore unsigned value in struct, use enum only in function (llvm#165048)

Typed enums are c23 features and are too new to be used. This PR
restores the types in the `__processor_model` struct back to `unsigned
int`, removes typed enums, and uses the enum in the function as a
variable that's later assigned to a struct in order to prevent errors
fixed initially here: llvm#164713

See llvm#165034 for more background

Author: mikolaj-pirog