[NFC][test] Mark the section which contains instructions executable

As we disscussed in D77971, we haven't confirmed that if putting instructions
in a non-executable section is an undefined behaviour. To make things
easier to go on, we mark these sections executable in test file
align-branch-section-size.s.

Author: KanRobert

llvm/test/MC/X86/align-branch-section-size.s

@@ -5,16 +5,16 @@
 
   # CHECK:  Name: text1
   # CHECK:  AddressAlignment: 32
-  .section text1
+  .section text1, "ax"
 foo:
   ret
 
   # CHECK:  Name: text2
   # CHECK:  AddressAlignment: 1
-  .section text2
+  .section text2, "ax"
   nop
 
   # CHECK:  Name: text3
   # CHECK:  AddressAlignment: 1
-  .section text3
+  .section text3, "ax"
   jmp foo