[arch][tests] add a way to query some arch mmu features

Allow asking the arch layer if it supports NX pages or NS pages.
Have the arch mmu test code test accordingly.
Also tweak the tests to pass on arm32 mmu, which does not precisely
match the return semantics of the rest of the mmu routines on map/unmap.

arch/riscv/mmu.cpp

@@ -399,15 +399,20 @@ int arch_mmu_map(arch_aspace_t *aspace, const vaddr_t _vaddr, paddr_t paddr, uin
     DEBUG_ASSERT(aspace);
     DEBUG_ASSERT(aspace->magic == RISCV_ASPACE_MAGIC);
 
-    if (count == 0) {
-        return NO_ERROR;
+    if (flags & ARCH_MMU_FLAG_NS) {
+        return ERR_INVALID_ARGS;
     }
+
     // trim the vaddr to the aspace
     if (_vaddr < aspace->base || _vaddr > aspace->base + aspace->size - 1) {
         return ERR_OUT_OF_RANGE;
     }
     // TODO: make sure _vaddr + count * PAGE_SIZE is within the address space
 
+    if (count == 0) {
+        return NO_ERROR;
+    }
+
     // construct a local callback for the walker routine that
     // a) tells the walker to build a page table if it's not present
     // b) fills in a terminal page table entry with a page and tells the walker to start over
@@ -591,6 +596,9 @@ void arch_mmu_context_switch(arch_aspace_t *aspace) {
     // for now, riscv_set_satp() does a full local TLB dump
 }
 
+bool arch_mmu_supports_nx_mappings(void) { return true; }
+bool arch_mmu_supports_ns_mappings(void) { return false; }
+
 extern "C"
 void riscv_mmu_init_secondaries() {
     // switch to the proper kernel pgtable, with the trampoline parts unmapped
@@ -634,6 +642,4 @@ void riscv_mmu_init() {
     printf("RISCV: MMU ASID mask %#lx\n", riscv_asid_mask);
 }
 
-
-
 #endif