Replace the PMC class struct copy with an explicit memcpy()

This should be effectively a nop for all archs, but for some reason the codegen
difference on the PowerPC 970 is such that the struct assignment doesn't work
(unless a printf() using one of the elements in the copied struct follows it),
while the memcpy() succeeds. On all archs the memcpy() should be expanded to an
inline copy, since the copy is bounded to ~16 bytes.

MFC after: 3 weeks