A built kernel can contain build paths:

$ strings bzImage-5.0.19-yocto-standard |grep tmp/
out of pgt_buf in /data/poky-tmp/reproducible/tmp/work-shared/qemux86-64/kernel-source/arch/x86/boot/compressed/kaslr_64.c!?

Easy enough to find that in the kernel:

$ git grep 'out of pgt_buf'
arch/x86/boot/compressed/kaslr_64.c:            debug_putstr("out of pgt_buf in " __FILE__ "!?\n");

But what's this in the top-level Makefile:

$ git grep prefix-map
Makefile:KBUILD_CFLAGS  += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)

So the __FILE__ shouldn't be using the full path.  However arch/x86/boot/compressed/Makefile has this:

KBUILD_CFLAGS := -m$(BITS) -O2

So that clears KBUILD_FLAGS, removing the -fmacro-prefix-map option.

I've now hit the edge of my knowledge.  Is the build in arch/x86/boot/compressed sufficiently special that it *should* start from a clean KBUILD_CFLAGS, so it needs this added:

+KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)

Or, should it *not* clear the KBUILD_CFLAGS and instead just extend or prune.  Of not is that none of the other architectures reset KBUILD_CFLAGS but prune it first, so I'm guessing that is what the x86 Makefile should do too.