Add some type safety to other pagemap accesses.

Introduce a descriptor for the pagemap config and check that the source
descriptor is compatible with the destination type.

src/test/func/two_alloc_types/alloc1.cc

@@ -4,6 +4,7 @@
 #define USE_RESERVE_MULTIPLE 1
 #define NO_BOOTSTRAP_ALLOCATOR
 #define IS_ADDRESS_SPACE_CONSTRAINED
+#define SNMALLOC_EXPOSE_PAGEMAP
 #define SNMALLOC_NAME_MANGLE(a) enclave_##a
 // Redefine the namespace, so we can have two versions.
 #define snmalloc snmalloc_enclave

src/test/func/two_alloc_types/alloc2.cc

@@ -1,6 +1,7 @@
 #undef IS_ADDRESS_SPACE_CONSTRAINED
 #define SNMALLOC_NAME_MANGLE(a) host_##a
 #define NO_BOOTSTRAP_ALLOCATOR
+#define SNMALLOC_EXPOSE_PAGEMAP
 // Redefine the namespace, so we can have two versions.
 #define snmalloc snmalloc_host
-#include "../../../override/malloc.cc"
+#include "../../../override/malloc.cc"

src/test/func/two_alloc_types/main.cc

@@ -32,6 +32,11 @@ extern "C" void host_free(void*);
 extern "C" void* enclave_malloc(size_t);
 extern "C" void enclave_free(void*);
 
+extern "C" void*
+enclave_snmalloc_get_global_pagemap(snmalloc::PagemapConfig const**);
+extern "C" void*
+host_snmalloc_get_global_pagemap(snmalloc::PagemapConfig const**);
+
 using namespace snmalloc;
 int main()
 {
@@ -42,6 +47,10 @@ int main()
   oe_end = (uint8_t*)oe_base + size;
   std::cout << "Allocated region " << oe_base << " - " << oe_end << std::endl;
 
+  // Call these functions to trigger asserts if the cast-to-self doesn't work.
+  enclave_snmalloc_get_global_pagemap(nullptr);
+  host_snmalloc_get_global_pagemap(nullptr);
+
   auto a = host_malloc(128);
   auto b = enclave_malloc(128);