[arch][mmu] add another routine to query if the arch supports user aspaces

Trim the arch mmu unit tests accordingly.

Should probably switch this to a #define, but it's possible some of
these queries could be dynamically detected (XN for example). May
revisit at some point.

arch/riscv/mmu.cpp

@@ -598,6 +598,7 @@ void arch_mmu_context_switch(arch_aspace_t *aspace) {
 
 bool arch_mmu_supports_nx_mappings(void) { return true; }
 bool arch_mmu_supports_ns_mappings(void) { return false; }
+bool arch_mmu_supports_user_aspaces(void) { return true; }
 
 extern "C"
 void riscv_mmu_init_secondaries() {