[sgen] Never invoke libc’s memmove() or bzero().

Those two functions are not guaranteed to do word-atomic copying or
zeroing under any circumstances.  memmove() on OSX, for instance,
has an SSE optimized variant that, if source and destination are 8-byte,
but not 16-byte aligned, will copy the first 8 bytes byte-wise, which can
crash SGen.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=16449.