kernel: use iremap for assembly

kernel: use iremap for assembly

Currently assembly code of the Kernel may contain the build path, resulting in
unreproducible firmware images (and Kernels). Fix this by adding a iremap
KAFLAG.

Before:

    strings "/path/to/vmlinux" | grep linux-6.18.38
    /path/to/openwrt/build_dir/target-x86_64_musl/linux-x86_64/linux-6.18.38
    /path/to/openwrt/build_dir/target-x86_64_musl/linux-x86_64/linux-6.18.38

After

    strings "/path/to/vmlinux" | grep linux-6.18.38
    target-x86_64_musl/linux-x86_64/linux-6.18.38
    target-x86_64_musl/linux-x86_64/linux-6.18.38

Link: https://github.com/openwrt/openwrt/pull/24271
Signed-off-by: Paul Spooren <mail@aparcar.org>