cmd/5a: prepare reglist correctly

cmd/5a: prepare reglist correctly

This was working when 5a was built on x86 because REG_R0 = 32,
and a 32-bit shift on x86 uses only the low 32 bits of the shift count.

On ARM, the shift clamping is different.

Moving to Go will avoid these differing shift semantics.
I tripped over and fixed this bug in new5a the same way earlier tonight.

Change-Id: Id56aa0bb1830ccf250960f843e0acb8a0409e87d
Reviewed-on: https://go-review.googlesource.com/3961
Reviewed-by: Dave Cheney <dave@cheney.net>