[6.2] Embedded Wasm: enable `import WASILibc` (#83846)
**Explanation**: Cherry-pick of #83792, merged as 450cb145971cd9c39dbad802f012d8cc06efc2a7
WASILibc wasn't built for the embedded stdlib in `stdlib/public/Platform/CMakeLists.txt`. New `copy_wasilibc_modulemap_embedded_resource` and `embedded-stdlib-platform-${triple}` targets are added, the latter for `wasm32-unknown-wasip1` only for now. Also added a `wasilibc_functions.swift` test to verify the result.
**Scope**: limited to Embedded Swift for Wasm;
**Risk**: low due to limited scope;
**Testing**: added new lit test to the existing Embedded Swift test suite;
**Issue**: rdar://157467412
**Reviewer**: @bnbarham
