Fix R15 asm error when build in dynamic link mode

[MXWXZ]

Fix #54, When imported and build with -buildmode=plugin or anything use dynamic link, go will throw

when dynamic linking, R15 is clobbered by a global variable access and is used here

ref: review src issue

[greatroar]

Looks like SI and DI are free, why not use one of those?

[cespare]

Thanks, but I'd like to fix this a different way. See #57.