community/mpir: enable ppc64le

mpir contains assembly code for some architectures, as well as a generic implementation to ensure build succeeds if hand-optimized assembly code is not available for a particular arch. The generic implementation is used for e.g. riscv64, loongarch64 and s390x. The build attempts to use powerpc64 assembly for ppc64le arch, but it is written for ABIv1, while Alpine uses ABIv2.

This MR forces the ppc64le build to use the generic code instead of assembly, allowing us to enable support for this platform with a slight performance hit.