[gdb/testsuite] Add missing .note.GNU-stack in gdb.arch/amd64-disp-step-self-call.exp

For test-case gdb.arch/amd64-disp-step-self-call.exp I get:
...
gdb compile failed, ld: warning: amd64-disp-step-self-call0.o: \
  missing .note.GNU-stack section implies executable stack
ld: NOTE: This behaviour is deprecated and will be removed in a future \
  version of the linker
...

Fix this by adding the missing .note.GNU-stack.

Likewise for gdb.arch/i386-disp-step-self-call.exp.

Tested on x86_64-linux.