kbuild: do not generate .tmp_vmlinux*.map when CONFIG_VMLINUX_MAP=y

Commit 5cc12472 ("kbuild: add CONFIG_VMLINUX_MAP expert option")
mentioned that "the .map file can be rather large (several MB), and
that's a waste of space when one isn't interested in these things."

If that is the case, generating map files for the intermediate
tmp_vmlinux* files is also a waste of space. It is unlikely that
anyone would be interested in the .tmp_vmlinux*.map files.

This commit stops passing the -Map= option when linking the .tmp_vmlinux*
intermediates.

I also hard-coded the file name 'vmlinux.map' instead of ${output}.map
because a later commit will introduce vmlinux.unstripped but I want to
keep the current name of the map file.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>