[gdb/testsuite] Fix gdb.base/nested-subp{2,3}.exp with recent ld

On openSUSE Tumbleweed (with ld 2.39) I get for test-case
gdb.base/nested-subp2.exp:
...
gdb compile failed, ld: warning: tmp.o: requires executable stack \
  (because the .note.GNU-stack section is executable)
...

Fix this by compiling with -Wl,--no-warn-execstack.

Likewise in gdb.base/nested-subp3.exp

Tested on x86_64-linux.