Build fail for emscripten profile

When I try to build for default or clang profile it works, but for emscripten profile it always fails.
Is it possible to add more `conan emscripten` recipes?

`conan install . -pr emscripten`

ERROR

```
newargs = [a for a in newargs if a is not '']
shared:ERROR: compiler frontend failed to generate LLVM bitcode, halting
make[2]: *** [source_subfolder/CMakeFiles/double-conversion.dir/build.make:78: source_subfolder/CMakeFiles/double-conversion.dir/double-conversion/bignum-dtoa.cc.o] Error 1
In file included from /home/horky/.conan/data/double-conversion/3.1.1/bincrafters/stable/build/6fbfc6b12a1688664056f827212be57d585d78c1/source_subfolder/double-conversion/double-conversion.cc:32:
In file included from /home/horky/.conan/data/double-conversion/3.1.1/bincrafters/stable/build/6fbfc6b12a1688664056f827212be57d585d78c1/source_subfolder/double-conversion/double-conversion.h:31:
/home/horky/.conan/data/double-conversion/3.1.1/bincrafters/stable/build/6fbfc6b12a1688664056f827212be57d585d78c1/source_subfolder/double-conversion/utils.h:110:2: error: Target architecture was not detected as supported by Double-Conversion.
#error Target architecture was not detected as supported by Double-Conversion.
^
1 error generated.
/home/horky/.conan/data/emsdk_installer/1.38.22/bincrafters/stable/package/83753ef0acbb6f750f6b8535092aae8d58e88098/emscripten/1.38.22/emcc.py:790: SyntaxWarning: "is not" with a literal. Did you mean "!="?
newargs = [arg for arg in newargs if arg is not '']
/home/horky/.conan/data/emsdk_installer/1.38.22/bincrafters/stable/package/83753ef0acbb6f750f6b8535092aae8d58e88098/emscripten/1.38.22/emcc.py:901: SyntaxWarning: "is not" with a literal. Did you mean "!="?
newargs = [a for a in newargs if a is not '']
shared:ERROR: compiler frontend failed to generate LLVM bitcode, halting
make[2]: *** [source_subfolder/CMakeFiles/double-conversion.dir/build.make:120: source_subfolder/CMakeFiles/double-conversion.dir/double-conversion/double-conversion.cc.o] Error 1
make[2]: Leaving directory '/home/horky/.conan/data/double-conversion/3.1.1/bincrafters/stable/build/6fbfc6b12a1688664056f827212be57d585d78c1/build_subfolder'
make[1]: *** [CMakeFiles/Makefile2:94: source_subfolder/CMakeFiles/double-conversion.dir/all] Error 2
make[1]: Leaving directory '/home/horky/.conan/data/double-conversion/3.1.1/bincrafters/stable/build/6fbfc6b12a1688664056f827212be57d585d78c1/build_subfolder'
make: *** [Makefile:130: all] Error 2
double-conversion/3.1.1@bincrafters/stable:
double-conversion/3.1.1@bincrafters/stable: ERROR: Package '6fbfc6b12a1688664056f827212be57d585d78c1' build failed
double-conversion/3.1.1@bincrafters/stable: WARN: Build folder /home/horky/.conan/data/double-conversion/3.1.1/bincrafters/stable/build/6fbfc6b12a1688664056f827212be57d585d78c1
ERROR: double-conversion/3.1.1@bincrafters/stable: Error in build() method, line 48
cmake.build()
ConanException: Error 2 while executing cmake --build '/home/horky/.conan/data/double-conversion/3.1.1/bincrafters/stable/build/6fbfc6b12a1688664056f827212be57d585d78c1/build_subfolder' '--' '-j8'
```

**conanfile.txt**

```
[requires]
opencv/4.1.1@conan/stable

[generators]
cmake

[options]
opencv:shared=True
```

**Profiles**

```
default
[settings]
os=Linux
os_build=Linux
arch=x86_64
arch_build=x86_64
compiler=gcc
compiler.version=9
compiler.libcxx=libstdc++
build_type=Release
[options]
[build_requires]
[env]
```

clang

```
[settings]
os=Linux
os_build=Linux
arch=x86_64
arch_build=x86_64
compiler=clang
compiler.version=6.0
compiler.libcxx=libstdc++
build_type=Release
[options]
[build_requires]
[env]
CC=/usr/bin/clang
CXX=/usr/bin/clang++
```

emscripten

```
include(default)
[settings]
os=Emscripten
arch=wasm
compiler=clang
compiler.version=6.0
compiler.libcxx=libc++

[options]
[build_requires]
emsdk_installer/1.38.22@bincrafters/stable

[env]
```

Start by running `conan install . -pr emscripten` with the shown `conanfile.txt` and emscripten profile. Read the profile and the failing double-conversion build output to determine which Conan recipe or platform support is missing; done means the emscripten dependency installation and build complete successfully.

Written by the indexing model from the issue text.