Rollup merge of #31985 - ashleysommer:libbackgrace_emscripten_fix, r=alexcrichton
Rollup merge of #31985 - ashleysommer:libbackgrace_emscripten_fix, r=alexcrichton

Was getting error:
```
running: "sh" "/home/flubba86/rust/src/libstd/../libbacktrace/configure" "--with-pic" "--disable-multilib" "--disable-shared" "--disable-host-shared" "--host=asmjs-unknown-emscripten" "--build=x86_64-unknown-linux-gnu"
...
Invalid configuration `asmjs-unknown-emscripten': system `emscripten' not recognized
```
This commit adds the emscripten target to the libbacktrace configure script so it is no longer unrecognized.

graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust