Use register numbers instead of register names

Avoid ambiguity by using register numbers. Register names are
interpreted depending on the ABI. Fixes the issue with mips64 gcc,
where registers t0-t4 are translated into $8-$11 (o32 ABI) instead
of $12-$15 (o64 ABI).

Also, use t9 ($25) for storing function address
(in getProcAddress.cpp).

Change-Id: I12ef5cd98bf14dab3c01f2756889cc02856c1cd8