Factor common userland and baremetal C functions

This allows add.c to run unmodified on both!

For that to work, use int main on baremetal, and pass the return value to
the final exit.

baremetal/lib/aarch64.S

@@ -8,5 +8,4 @@ mystart:
     ldr x0, =stack_top
     mov sp, x0
     bl main
-    mov x0, #0
     bl exit

baremetal/lib/arm.S

@@ -2,5 +2,4 @@
 mystart:
     ldr sp, =stack_top
     bl main
-    mov r0, #0
     bl exit

baremetal/lib/common.h

@@ -1,4 +0,0 @@
-#ifndef COMMON_H
-#define COMMON_H
-void assert_fail();
-#endif

baremetal/lib/syscalls.c

@@ -86,8 +86,3 @@ void _exit(int status) {
 #endif
 #endif
 }
-
-void assert_fail() {
-	puts("lkmc_test_fail");
-	exit(1);
-}