Use mimalloc allocator for static build

Which fixes performance problems, especially on heavily multi-threaded workloads (many functions, high core count machines), see #5561.

Author: danleh

.gitmodules

@@ -7,3 +7,6 @@
 [submodule "third_party/fuzztest"]
 	path = third_party/fuzztest
 	url = https://github.com/google/fuzztest
+[submodule "third_party/mimalloc"]
+	path = third_party/mimalloc
+	url = https://github.com/microsoft/mimalloc.git

CMakeLists.txt

@@ -446,6 +446,9 @@ if(BUILD_STATIC_LIB)
   message(STATUS "Building libbinaryen as statically linked library.")
   add_library(binaryen STATIC)
   add_definitions(-DBUILD_STATIC_LIBRARY)
+  
+  message(STATUS "Using mimalloc for static build to avoid bad musl libc allocator performance.")
+  target_link_libraries(binaryen mimalloc-static)
 else()
   message(STATUS "Building libbinaryen as shared library.")
   add_library(binaryen SHARED)

third_party/CMakeLists.txt

@@ -15,3 +15,7 @@ endif()
 if(BUILD_LLVM_DWARF)
   add_subdirectory(llvm-project)
 endif()
+
+if(BUILD_STATIC_LIB)
+  add_subdirectory(mimalloc)
+endif()