[scudo] Limit by default the TSD pool to 2 on Android

[cryptoad]

Summary

jemalloc on Android currently uses 2 arenas
(https://android.googlesource.com/platform/external/jemalloc/+/master/Android.bp#64).
Since the Android toolchain absorbs compiler-rt and compiles it as is, we have
to enforce the same limit to somehow stay competitive in terms of memory usage.

The changes could either go in:

• scudo_platform.h with a default for Android of 2 (this is the solution implemented here);
• in CMakeLists.txt adding -DSCUDO_SHARED_TSD_POOL_SIZE=2 for Android.
• something else?

I don't have a strong opinion on how to do it, but it has to be done upstream
anyway.

[alekseyshl]

Looks reasonable

lib/scudo/scudo_platform.h

 
 // If the exclusive TSD model is chosen, make sure the platform supports it.
 #if SCUDO_TSD_EXCLUSIVE && !SCUDO_TSD_EXCLUSIVE_SUPPORTED
 # error "The exclusive TSD model is not supported on this platform."
 #endif
 
 // Maximum number of TSDs that can be created for the Shared model.
 #ifndef SCUDO_SHARED_TSD_POOL_SIZE
-# define SCUDO_SHARED_TSD_POOL_SIZE 32U
+# if SANITIZER_ANDROID
+#  define SCUDO_SHARED_TSD_POOL_SIZE 2U
+# else
+#  define SCUDO_SHARED_TSD_POOL_SIZE 32U
+# endif  // SANITIZER_ANDROID
 #endif  // SCUDO_SHARED_TSD_POOL_SIZE
 
 // The following allows the public interface functions to be disabled.
 #ifndef SCUDO_CAN_USE_PUBLIC_INTERFACE
 # define SCUDO_CAN_USE_PUBLIC_INTERFACE 1
 #endif
 
 namespace __scudo {