[lld][WebAssembly] Fix for weak undefined functions in -pie mode

This fixes two somewhat related issues.  Firstly we were never
generating imports for weak functions (even with the `import-functions`
policy for undefined symbols).  Adding a direct call to foo in the
`weak-undefined-pic.s` exposed a crash in the linker which this
change fixes.

Secondly we were failing to call `handleWeakUndefines` for the `-pie`
case which is PIC but doesn't set the undefined symbol policy to
`import-functions`.  With this change `-pie` binaries will by default
call `handleWeakUndefines` which generates the undefined stub handlers
for any weakly undefined symbols.

Fixes: https://github.com/emscripten-core/emscripten/issues/13337

Differential Revision: https://reviews.llvm.org/D95914