runtime: load runtime.goarm as a byte, not a word

runtime: load runtime.goarm as a byte, not a word

Fixes #6952.

runtime.asminit was incorrectly loading runtime.goarm as a word, not a uint8 which made it subject to alignment issues on arm5 platforms.

Alignment aside, this also meant that the top 3 bytes in R11 would have been garbage and could not be assumed to be setting up the FPU reliably.

R=iant, minux.ma
CC=golang-codereviews
https://golang.org/cl/46240043