bpf-headers: fix compilation with LLVM_IAS=1

bpf-headers: fix compilation with LLVM_IAS=1

Linux 5.10.178 includes backported commits that break the compilation
of bpf-headers, as the compilation gets confused which assembler to use.
Caused by Linux upstream commits just before the .178 tag:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/log/?h=v5.10.178

2023-04-20	kbuild: check CONFIG_AS_IS_LLVM instead of LLVM_IAS
2023-04-20	kbuild: Switch to 'f' variants of integrated assembler flag
2023-04-20	kbuild: check the minimum assembler version in Kconfig

Explicitly use LLVM_IAS=1 to fix things.

Fixes #12748

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>