Fix build error when compiling MIPS SIMD w/ -mfpxx

Fix build error when compiling MIPS SIMD w/ -mfpxx

When compiled with -mfpxx (which is now the default on Debian), there are
some restrictions on the use of odd-numbered FP registers. More details
about FPXX can be found here:
https://dmz-portal.mips.com/wiki/MIPS_O32_ABI_-_FR0_and_FR1_Interlinking

This commit simply changes all uses of FP registers to an even-numbered
equivalent like this:
 f0 -> f0
 f1 -> f2
 f2 -> f4
 ...
 f8 -> f16

This commit should have no observable effect except that the MIPS assembly
will now compile with -mfpxx.

Closes #11