inline assembly: improve everywhere

userland/arch/aarch64/asm_hello.c

@@ -2,7 +2,12 @@
 #include <inttypes.h>
 
 int main(void) {
-    register uint32_t x0 __asm__ ("x0");
-    __asm__ ("mov x0, #1;" : : : "%x0");
-    assert(x0 == 1);
+    uint32_t myvar = 1;
+    __asm__ (
+        "add %[myvar], %[myvar], 1;"
+        : [myvar] "=r" (myvar)
+        :
+        :
+    );
+    assert(myvar == 2);
 }