Since commit f593b354 (PCH: Add support for multi architecture iOS
projects, 2020-04-02, v3.18.0-rc1~414^2) we use per-arch .pch files
even when compiling for just the host architecture on macOS arm64.
This breaks with compilers that do not support `-Xarch_` flags, such
as GCC.  Avoid using per-arch .pch files in single-architecture builds.

Fixes: #25514
Issue: #20497