middle-end/32667 - document cpymem and memcpy exact overlap requirement

middle-end/32667 - document cpymem and memcpy exact overlap requirement

The following amends the cpymem documentation to mention that exact
overlap needs to be handled gracefully, also noting that the target
runtime is expected to behave the same way where -ffreestanding
docs mention the set of routines required.

	PR middle-end/32667
	* doc/md.texi (cpymem): Document that exact overlap of source
	and destination needs to work.
	* doc/standards.texi (ffreestanding): Mention memcpy is required
	to handle the exact overlap case.