[embind] Fix and test for incorrect dynCalls signature (#24791)

Embind with C++ float types _and_ `-sDYNCALLS=1` (and by
extension `-sASYNCIFY=1`) fails with:
```
Assertion failed: bad function pointer type - sig is not in dynCalls: 'iid'
```
This adds a one-char fix (from @RReverser) and adds a regression test.

Fixes: #24762

Author: cwoffenden

system/include/emscripten/em_asm.h

@@ -149,7 +149,7 @@ void emscripten_asm_const_async_on_main_thread(
 // use std::tuple or std::integral_constant. Using C++11 features is only a
 // warning in modern Clang, which are ignored in system headers.
 template<typename, typename = void> struct __em_asm_sig {};
-template<> struct __em_asm_sig<float> { static const char value = 'd'; };
+template<> struct __em_asm_sig<float> { static const char value = 'f'; };
 template<> struct __em_asm_sig<double> { static const char value = 'd'; };
 template<> struct __em_asm_sig<char> { static const char value = 'i'; };
 template<> struct __em_asm_sig<signed char> { static const char value = 'i'; };

test/test_other.py

@@ -3356,6 +3356,8 @@ def test_embind_no_raw_pointers(self, filename):
     'o2_mem_growth': ['-O2', '-sALLOW_MEMORY_GROWTH', test_file('embind/isMemoryGrowthEnabled=true.cpp')],
     'o2_closure': ['-O2', '--closure=1', '--closure-args', '--externs ' + shlex.quote(test_file('embind/underscore-externs.js')), '-sASSERTIONS=1'],
     'strict_js': ['-sSTRICT_JS'],
+    # DYNCALLS tests the legacy native function API (ASYNCIFY implicitly enables DYNCALLS)
+    'dyncalls': ['-sDYNCALLS=1'],
   })
   def test_embind(self, *extra_args):
     if '-sMEMORY64' in extra_args: