um: Use CFLAGS_vmlinux

link-vmlinux.sh doesn't use LDFLAGS_vmlinux when linking the kernel for
UML. Move the LDFLAGS_EXESTACK options into CFLAGS_vmlinux so they're
actually respected.

e.g.
/usr/lib/gcc/x86_64-pc-linux-gnu/12/../../../../x86_64-pc-linux-gnu/bin/ld: warning: .tmp_vmlinux.kallsyms3.o: missing .note.GNU-stack section implies executable stack
/usr/lib/gcc/x86_64-pc-linux-gnu/12/../../../../x86_64-pc-linux-gnu/bin/ld: NOTE: This behaviour is deprecated and will be removed in a future version of the linker
/usr/lib/gcc/x86_64-pc-linux-gnu/12/../../../../x86_64-pc-linux-gnu/bin/ld: warning: vmlinux has a LOAD segment with RWX permissions

Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Peter Foley <pefoley2@pefoley.com>
Signed-off-by: Richard Weinberger <richard@nod.at>