Call __libc_init_malloc from scudo.

Currently, scudo doesn't call libc's malloc initialisers. This causes
problems with any functionality that relies on an initialised__libc_globals
inside of bionic malloc's stubs (e.g. malloc()).

This manifests in two ways (that I can think of):
 1. Dispatch tables don't work with scudo, so malloc_debug has never
 worked in an executable linked against scudo.
 2. Allocators that require initialisation and are called from bionic
 malloc's stubs (GWP-ASan) never get initialised.

Bug: 135634846
Test: atest bionic-unit-tests-scudo
Change-Id: I3e3344d7d510ce4e8d3709cd69c8cb0fe5adedda

libc/bionic/scudo_wrapper.cpp

@@ -35,6 +35,11 @@
 
 __LIBC_HIDDEN__ WriteProtected<libc_globals> __libc_globals;
 
+// Call the libc malloc initialisers.
+__attribute__((constructor(1))) static void __scudo_preinit() {
+  __libc_globals.mutate(__libc_init_malloc);
+}
+
 #if defined(__i386__)
 __LIBC_HIDDEN__ void* __libc_sysinfo = reinterpret_cast<void*>(__libc_int0x80);
 #endif