wasm-emscripten-finalize: Improve shared library support (#1961)

Convert PIC code generated by llvm to work with the current emscripten
ABI for dynamic linking:

- Convert mutable global imports from GOT.mem and GOT.func into
  internal globals.
- Initialize these globals on started up in g$foo and f$foo imported functions
  to calculate addresses at runtime.

Also:
- Add a test case for linking and finalizing a shared library
- Allow __stack_pointer global to be non-existent as can be case for
  a shared library.
- Allow __stack_pointer global to be an import, as can be the case for
  a shared library.

1 files changed, 7 insertions, 0 deletions

diff --git a/test/lld/shared.c b/test/lld/shared.c
new file mode 100644
index 000000000..1d98b068b
--- /dev/null
+++ b/test/lld/shared.c
@@ -0,0 +1,7 @@
+int puts(const char* str);
+extern int external_var;
+
+int print_message() {
+  puts("Hello, world");
+  return external_var;
+}