x86 asm: align stack to 16-bits for abort() call

Fixes the failing tests that called abort(). For coincidence, only native
tests were failing.

Asked at: https://stackoverflow.com/questions/56324948/why-does-calling-the-c-abort-function-from-an-x86-64-assembly-function-lead-to

lkmc/x86_64.h

@@ -31,6 +31,7 @@
  * https://github.com/cirosantilli/linux-kernel-module-cheat#x86_64-calling-convention
  */
 #define LKMC_EPILOGUE \
+    add $8, %rsp; \
     pop %rbx; \
     pop %r12; \
     pop %r13; \
@@ -56,6 +57,7 @@ main: \
     push %r13; \
     push %r12; \
     push %rbx; \
+    sub $8, %rsp; \
 main_after_prologue: \
 ;