Porting to non-x86 (esp. aarch64)

[corwin-of-amber]

This PR addresses two portability issues:

• The setting or un-setting of -DTIMING was not respected everywhere. Since the timing primitives use x86 assembly, this was a blocker.
• Variadic function calls follow a different convention from fixed-parameter ones on ARM processors. So function pointers of type void *(*)(elina_manager_t*,...) were replaced with their specific counterparts.

Fixes #88.

[corwin-of-amber]

Note: I carried out the second item in two passes to make reviewing the changes easier.