core/mem: Add zero_explicit

This call is intended to provide the ability to securely scrub memory
without compiler interference, in a similar manner to explicit_bzero,
memset_s, SecureZeroMemory.

The approach taken is a volatile memset followed by a seqentially
consistent memory fence, to prevent the call from being optimized away
by DSE, and from being reordered.  An identical approach is currently
being used by the zeroize Rust crate, and is effective in practice.

LLVM IR output:
```
; Function Attrs: nounwind
define internal i8* @mem.zero_explicit(i8* %0, i64 %1) #0 {
decls:
  call void @llvm.memset.p0i8.i64(i8* %0, i8 0, i64 %1, i1 true)
  fence seq_cst
  ret i8* %0
}
```

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