libunwindstack: don't save pseudoregisters while evaluating Dwarf

Currently, while evaluating a Dwarf section, even pseudoregisters are
saved in regs_info. Since pseudoregisters are stored separately from
ordinary registers, trying to read them the usual way will result in
an out-of-bounds read. There's no memory corruption as regs_info is
big enough to store all existing pseudoregisters.

With this patch, pseudoregisters are simply not saved in regs_info.

Added new unit tests to cover the pseudo register cases.

Test: libunwindstack_test

Change-Id: If21b2a79f2fcca85644eec430f3d22e354b001ec