Emscripten target spurious internal compiler errors (corrupted double-linked list)

[kvark]

(I'm not completely sure if it's a Rust bug in the first place, but previously we've had issues with emscripten target linking/compiling, and they were resolved on Rust side, so here we come again).

You can see the error happening in this travis job:

Error in `/emsdk_portable/llvm/clang/bin/llc': corrupted double-linked list: 0x000000000379a1e0 ***

According to gfx-rs/gfx#2408, it happens on both wasm32 and asmjs targets, and the error may look different:

• corrupted size vs. prev_size
• double free or corruption (!prev)

[jonas-schievink]

Duplicate of #47023, as far as I can tell