scripts/Makefile.autoconf: use abs_srctree for out-of-tree symlink

scripts/Makefile.autoconf: use abs_srctree for out-of-tree symlink

Replace usage of $(srctree) with $(abs_srctree) when creating a symlink
to include/asm/arch in out of tree builds.

When building_out_of_srctree is true, $(srctree) is just "..", so the
created symlink was broken, for example:

    build-mt8365_evk/include/asm/arch -> ../arch/arm/include/asm/arch-mediatek

Which would resolve to a non-existent path:

    build-mt8365_evk/include/asm/arch/arm/include/asm/arch-mediatek

To fix, we need to use the absolute path to the source tree since we
don't know where the build tree is located relative to the source tree.

Fixes: bd3f9ee679b4 ("kbuild: Bump the build system to 6.1")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Tested-by: Sean Anderson <sean.anderson@linux.dev>