NFC: Feed Pagemap its primitive allocator as template arg

This will let us use Pagemaps further down the dependency stack (specifically,
we're going to want a Pagemap inside the AddressSpaceManager) by letting us
manually tie the knot rather than rely on GlobalVirtual and
default_memory_provider() being defined by the time we want a Pagemap.

src/test/func/pagemap/pagemap.cc

@@ -15,7 +15,7 @@ using namespace snmalloc;
 using T = size_t;
 static constexpr size_t GRANULARITY_BITS = 9;
 static constexpr T PRIME = 251;
-Pagemap<GRANULARITY_BITS, T, 0> pagemap_test;
+Pagemap<GRANULARITY_BITS, T, 0, DefaultPrimAlloc> pagemap_test;
 
 int main(int argc, char** argv)
 {