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

README.adoc

@@ -11998,6 +11998,8 @@ Examples:
 
 * link:lkmc/x86_64.h[] `ENTRY` and `EXIT`
 
+One important catch is that the stack must always be aligned to 16-bits before making calls: https://stackoverflow.com/questions/56324948/why-does-calling-the-c-abort-function-from-an-x86-64-assembly-function-lead-to
+
 Bibliography:
 
 * https://en.wikipedia.org/wiki/X86_calling_conventions#System_V_AMD64_ABI
@@ -13434,7 +13436,7 @@ output:
 
 ....
 CPSR.M 0x3
-CPSR.M 0x3
+CPSR.M 0xA
 CPSR.M 0x3
 CurrentEL.EL 0x1
 CurrentEL.EL 0x2

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: \
 ;