Fix some paging related bugs exposed by the spawn stress test.

- Process::exec() needs to restore the original paging scope when called
  on a non-current process.
- Add missing InterruptDisabler guards around g_processes access.
- Only flush the TLB when modifying the active page tables.

AK/Compiler.h

@@ -3,7 +3,7 @@
 #define PACKED __attribute__ ((packed))
 #define NORETURN __attribute__ ((noreturn))
 #undef ALWAYS_INLINE
-#define ALWAYS_INLINE __attribute__ ((always_inline))
+#define ALWAYS_INLINE inline __attribute__ ((always_inline))
 #define NEVER_INLINE __attribute__ ((noinline))
 #define MALLOC_ATTR __attribute__ ((malloc))
 #define PURE __attribute__ ((pure))