As of Clang 18, calling memcpy() with a misaligned pointer trips UBSAN,
even if the length is zero. This unfortunately includes any call to
`@memcpy` when source or destination are undefined and the length is
zero.

This patch makes the C backend avoid calling memcpy when the length is
zero, thereby avoiding undefined behavior.

A zig1.wasm update will be needed in the llvm18 branch to activate this
code.