wasm/baremetal lit tests failing on NixOS

[kinke]

First of all, these errors are not crucial.

Issue 1: tests/baremetal/wasm.d shouldn't explicitly use 
`-link-internally`, as that requires LDC to be built with CMake 
LDC_WITH_LLD != OFF *and* installed LLD dev files *and* that LLD 
to be patched (https://github.com/ldc-developers/lld) to work 
correctly.

Issue 2: both wasm.d and wasm2.d expect linking to WebAssembly to 
work, and that is currently only achievable via an integrated 
patched LLD. For this to work with an external linker (e.g., 
LLD's `wasm-ld`), LDC needs to be fixed by invoking the linker 
directly for WebAssembly targets, instead of issuing a gcc/clang 
command line as it currently does.