High registers (r8+) cannot be used

[mattlgy]

A change to rustc, in how instruction_set and asm interact, broke some uses of higher registers. Example error:

error: invalid register `r8`: high registers (r8+) cannot be used in Thumb-1 code
   --> .../gba/src/sync/statics.rs:100:59
    |
100 |       out("r2") _, out("r3") _, out("r4") _, out("r5") _, out("r8") _,
    | 

See https://github.com/rust-console/gba/pull/157 for some more discussion.