Now we start building the armv7hl kernel for Leap 15.4, and it fails as:

[ 9793s]   GEN     modules.builtin
[ 9793s]   LD      .tmp_vmlinux.btf
[ 9827s]   BTF     .btf.vmlinux.bin.o
[ 9884s] [4957384] STRUCT thread_info's field 'vfpstate' offset=2048 bit_size=0
type=4957356 Error emitting field
[ 9884s] Encountered error while encoding BTF.
....
[ 9994s] FAILED: load BTF from vmlinux: No such file or directory
[ 9994s] make[1]: ***
[/home/abuild/rpmbuild/BUILD/kernel-default-5.14.2/linux-5.14/Makefile:1200:
vmlinux] Error 255

My initial attempt worked because BTF wasn't enabled due to the old tools.
OTOH, the same 5.14 build is OK with stable branch, so there is still some
missing piece, supposedly.