[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/x86/32/mmu.c

@@ -405,6 +405,7 @@ int arch_mmu_map(arch_aspace_t * const aspace, const vaddr_t vaddr, const paddr_
 
 bool arch_mmu_supports_nx_mappings(void) { return false; }
 bool arch_mmu_supports_ns_mappings(void) { return false; }
+bool arch_mmu_supports_user_aspaces(void) { return false; }
 
 void x86_mmu_early_init(void) {
     /* Set WP bit in CR0*/

arch/x86/64/mmu.c

@@ -604,6 +604,7 @@ int arch_mmu_map(arch_aspace_t *const aspace, const vaddr_t vaddr, const paddr_t
 
 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 false; }
 
 void x86_mmu_early_init(void) {
     volatile uint64_t efer_msr, cr0, cr4;