dynlib keeps exported functions alive in emscripten (#23469)

ref https://forum.nim-lang.org/t/11338

ref https://github.com/beef331/wasm3/blob/master/src/wasm3/exporter.nim

ref emscripten-core/emscripten#6233

ref
https://github.com/emscripten-core/emscripten/blob/3.1.56/system/include/emscripten/em_macros.h#L10

`EMSCRIPTEN_KEEPALIVE` is a macro that is used to prevent unused
functions from being deadcode eliminated in emscripten, which is a
simple wrapper around `__attribute__((used))`. This PR follows suits and
expects dynlib is supposed to keep these functions alive. In the future,
`exportwasm` might be introduced.

After this PR, a function with `{.dynlib, exportc.}` can be reused from
other wasm programs reliably.

lib/nimbase.h

@@ -14,6 +14,7 @@ __GNUC__
 __TINYC__
 __clang__
 __AVR__
+__EMSCRIPTEN__
 */
 
 
@@ -177,12 +178,13 @@ __AVR__
 # define N_THISCALL_PTR(rettype, name) rettype (__thiscall *name)
 # define N_SAFECALL_PTR(rettype, name) rettype (__stdcall *name)
 
-# ifdef __cplusplus
-# define N_LIB_EXPORT NIM_EXTERNC __declspec(dllexport)
+# ifdef __EMSCRIPTEN__
+# define N_LIB_EXPORT NIM_EXTERNC __declspec(dllexport) __attribute__((used))
+# define N_LIB_EXPORT_VAR __declspec(dllexport) __attribute__((used))
 # else
 # define N_LIB_EXPORT NIM_EXTERNC __declspec(dllexport)
+# define N_LIB_EXPORT_VAR __declspec(dllexport)
 # endif
-# define N_LIB_EXPORT_VAR __declspec(dllexport)
 # define N_LIB_IMPORT extern __declspec(dllimport)
 #else
 # define N_LIB_PRIVATE __attribute__((visibility("hidden")))
@@ -211,8 +213,13 @@ __AVR__
 # define N_FASTCALL_PTR(rettype, name) rettype (*name)
 # define N_SAFECALL_PTR(rettype, name) rettype (*name)
 # endif
-# define N_LIB_EXPORT NIM_EXTERNC __attribute__((visibility("default")))
-# define N_LIB_EXPORT_VAR __attribute__((visibility("default")))
+# ifdef __EMSCRIPTEN__
+# define N_LIB_EXPORT NIM_EXTERNC __attribute__((visibility("default"), used))
+# define N_LIB_EXPORT_VAR __attribute__((visibility("default"), used))
+# else
+# define N_LIB_EXPORT NIM_EXTERNC __attribute__((visibility("default")))
+# define N_LIB_EXPORT_VAR __attribute__((visibility("default")))
+# endif
 # define N_LIB_IMPORT extern
 #endif