[web] Fix Web SDK build on macOS (flutter#183549)

Before this, I was getting a bunch of errors like this while trying to
run `felt build` on my Macbook.

```
../../flutter/buildtools/mac-arm64/clang/bin/clang++ -MMD -MF clang_arm64/obj/flutter/fml/fml.endianness.o.d -DUSE_OPENSSL=1 -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D_FORTIFY_SOURCE=2 -D_LIBCPP_DISABLE_AVAILABILITY=1 -D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS -D_LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DFLUTTER_RUNTIME_MODE_DEBUG=1 -DFLUTTER_RUNTIME_MODE_PROFILE=2 -DFLUTTER_RUNTIME_MODE_RELEASE=3 -DFLUTTER_RUNTIME_MODE_JIT_RELEASE=4 -DDART_LEGACY_API=\[\[deprecated\]\] -DFLUTTER_RUNTIME_MODE=1 -DFLUTTER_JIT_RUNTIME=1 -I../.. -Iclang_arm64/gen -I../../flutter/third_party/libcxxabi/include -I../../flutter/build/secondary/flutter/third_party/libcxx/config -I../../flutter -I../../third_party/abseil-cpp -fno-strict-aliasing -fstack-protector-all --target=arm64-apple-macos -arch arm64 -fcolor-diagnostics -Wall -Wextra -Wendif-labels -Wno-missing-field-initializers -Wno-unused-parameter -Werror -Wno-unused-but-set-parameter -Wno-unused-but-set-variable -Wno-psabi -Wno-implicit-int-float-conversion -Wno-deprecated-copy -Wno-deprecated-literal-operator -Wno-unqualified-std-cast-call -Wno-nontrivial-memcall -Wno-non-c-typedef-for-linkage -Wno-range-loop-construct -Wunguarded-availability -fdebug-prefix-map=/Users/het/Projects/flutter/engine/src/= -no-canonical-prefixes -fvisibility=hidden -Wstring-conversion -Wnewline-eof -O2 -fno-ident -fdata-sections -ffunction-sections -flto -g2 -Wunreachable-code -fvisibility-inlines-hidden -std=c++20 -Wno-deprecated-this-capture -fno-modules -Xclang -fno-cxx-modules -fno-rtti -nostdinc++ -I../../flutter/third_party/libcxx/include -I../../flutter/third_party/libcxxabi/include -nostdinc++ -fvisibility=hidden -fno-exceptions -stdlib=libc++ -isysroot ../../flutter/prebuilts/SDKs/MacOSX15.5.sdk -mmacosx-version-min=  -c ../../flutter/fml/endianness.cc -o clang_arm64/obj/flutter/fml/fml.endianness.o
clang++: error: missing version number in '-mmacosx-version-min='
```

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Author: harryterkelsen

engine/src/flutter/tools/gn

@@ -898,6 +898,9 @@ def to_gn_wasm_args(args, gn_args):
   gn_args['skia_canvaskit_profile_build'] = args.runtime_mode == 'profile'
   gn_args['flutter_prebuilt_dart_sdk'] = True
 
+  if get_host_os() == 'mac':
+    gn_args.update(setup_apple_sdks())
+
 
 def parse_args(args):
   args = args[1:]