Make copystr() a MI C function, part of libkern and shared on all
architectures.

Notes:

- On alpha and ia64 the function is kept but gets renamed locally to avoid
symbol collision. This is beca

Make copystr() a MI C function, part of libkern and shared on all
architectures.

Notes:

- On alpha and ia64 the function is kept but gets renamed locally to avoid
symbol collision. This is because on these two arches, I am not sure
whether the ASM callers do not rely on fixed registers, so I prefer to
keep the ASM body for now.
- On Vax, only the symbol is removed, because the body is used from other
functions.
- On RISC-V, this change fixes a bug: copystr() was just a wrapper around
strlcpy(), but strlcpy() makes the operation less safe (strlen on the
source beyond its size).
- The kASan, kCSan and kMSan wrappers are removed, because now that
copystr() is in C, the compiler transformations are applied to it,
without the need for manual wrappers.

Could test on amd64 only, but should be fine.

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