Add support for building ICU4C with ARM and ARM64 (non-UWP)

I added support for UWP ARM64 in https://unicode-org.atlassian.net/browse/ICU-20712

I've been working on adding support for ARM/ARM for non-UWP in a branch in my fork, and would like to use this ticket in order to keep the changes separate.

The changes are almost ready to merge.