LLVM_WASM: Support linking via emscripten

[ubaidsk]

No description provided.

[ubaidsk]

Steps:

1. Download and install emscripten from https://emscripten.org/docs/getting_started/downloads.html
2. Build with the following:

diff --git a/build1.sh b/build1.sh
index 49a1cf78b..24737c919 100755
--- a/build1.sh
+++ b/build1.sh
@@ -3,6 +3,8 @@
 set -e
 set -x
 
+export EMSDK_PATH=$HOME/ext/emsdk
+
 cmake \
     -DCMAKE_BUILD_TYPE=Debug \
     -DWITH_LLVM=yes \
@@ -12,5 +14,6 @@ cmake \
     -DCMAKE_PREFIX_PATH="$CMAKE_PREFIX_PATH_LFORTRAN;$CONDA_PREFIX" \
     -DCMAKE_INSTALL_PREFIX=`pwd`/inst \
     -DCMAKE_INSTALL_LIBDIR=share/lfortran/lib \
+    -DWITH_TARGET_WASM=yes \
     .
 cmake --build . -j16 --target install

3. Test using

% lfortran examples/expr2.f90 --target=wasm32-unknown-emscripten
25
% ls | grep expr2
expr2.out
expr2.out.tmp.o
expr2.wasm
% node expr2.out
25
% rm expr2*
% lfortran examples/expr2.f90 --target=wasm32-unknown-emscripten --wasm-html
25
% ls | grep expr2                                               
expr2.out.html
expr2.out.js
expr2.out.tmp.o
expr2.out.wasm
% node expr2.out.js
25
% python -m http.server
Serving HTTP on :: port 8000 (http://[::]:8000/) ...

In browser, you can open expr2.out.html to see

[ubaidsk]

Example from #3823 (comment) in browser looks as follows:

[certik]

I think this looks good. Thanks!

[certik]

Is non-emscripten WASM still supported?

[ubaidsk]

Is non-emscripten WASM still supported?

The one from clang (for wasi)? Yes that is still supported.