default to "preemptive" on wasm

It isn't really preemptive, it just turns off safepoint code in the AOT
compiler.  This is fine for single threaded.

Author: lambdageek

src/mono/CMakeLists.txt

@@ -546,6 +546,8 @@ elseif(GC_SUSPEND STREQUAL "default")
     set(ENABLE_COOP_SUSPEND 1)
   elseif(TARGET_SYSTEM_NAME STREQUAL "Windows")
     # use preemptive
+  elseif(TARGET_SYSTEM_NAME STREQUAL "Emscripten")
+    # use preemptive
   else()
     set(ENABLE_HYBRID_SUSPEND 1)
   endif()

src/mono/mono.proj

@@ -51,6 +51,8 @@
   <!-- default thread suspend for specific platforms -->
   <PropertyGroup>
     <MonoThreadSuspend Condition="'$(TargetswatchOS)' == 'true' and '$(MonoThreadSuspend)' == ''">coop</MonoThreadSuspend>
+    <!-- wasm isn't really preemptive, but we don't want safepoints -->
+    <MonoThreadSuspend Condition="'$(TargetsBrowser)' == 'true' and '$(MonoThreadSuspend)' == ''">preemptive</MonoThreadSuspend>
     <!-- all other platforms -->
     <MonoThreadSuspend Condition="'$(MonoThreadSuspend)' == ''">hybrid</MonoThreadSuspend>
   </PropertyGroup>