Clarify what abort_on_misalignment() does.

Clarify what abort_on_misalignment() does.

It doesn't request byte misalignment repair, it requests that byte
misalignment kill the program with SIGBUS; on platforms that don't
support aligned loads, we should be fetching possibly-misaligned data
using some safe instruction sequence, not by doing misaligned loads and
relying on them to trap to the kernel and be (slowly) emulated.