kbuild: tolerate missing pahole when generating BTF

When BTF generation is enabled through CONFIG_DEBUG_INFO_BTF,
scripts/link-vmlinux.sh detects if pahole version is too old and
gracefully continues build process, skipping BTF generation build step.
But if pahole is not available, build will still fail. This patch adds
check for whether pahole exists at all and bails out gracefully, if not.

Cc: Alexei Starovoitov <ast@fb.com>
Reported-by: NYonghong Song <yhs@fb.com>
Fixes: e83b9f55 ("kbuild: add ability to generate BTF type info for vmlinux")
Signed-off-by: NAndrii Nakryiko <andriin@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

scripts/link-vmlinux.sh

@@ -96,6 +96,11 @@ gen_btf()
 {
 	local pahole_ver;
 
+	if ! [ -x "$(command -v ${PAHOLE})" ]; then
+		info "BTF" "${1}: pahole (${PAHOLE}) is not available"
+		return 0
+	fi
+
 	pahole_ver=$(${PAHOLE} --version | sed -E 's/v([0-9]+)\.([0-9]+)/\1\2/')
 	if [ "${pahole_ver}" -lt "113" ]; then
 		info "BTF" "${1}: pahole version $(${PAHOLE} --version) is too old, need at least v1.13"