Fixes for macOS

[barracuda156]

This PR:

1. Adds initial support for PPC.
2. Fixes the build on macOS.
3. Makes minor fixes to makefiles: there is some inconsistency with targets, like non-existent folders being added etc.

What still needs to be done:

1. Implement tests for PPC specifically or platform-independent (there is no plain folder in tests now).
2. Fix dylib target for macOS: current target uses Linux-only linker flags.
3. What is the status of tool target? Is it broken, WIP or why is it disabled (and incomplete)?

[barracuda156]

@NCIC-AlphaSparse @xupinjie @JacoCheung Could someone look into this, please? Thank you!