bpf: Generate BTF_KIND_FLOAT when linking vmlinux

pahole v1.21 supports the --btf_gen_floats flag, which makes it
generate the information about the floating-point types [1].

Adjust link-vmlinux.sh to pass this flag to pahole in case it's
supported, which is determined using a simple version check.

[1] https://lore.kernel.org/dwarves/YHRiXNX1JUF2Az0A@kernel.org/Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210413190043.21918-1-iii@linux.ibm.com

scripts/link-vmlinux.sh

@@ -213,6 +213,7 @@ vmlinux_link()
 gen_btf()
 {
 	local pahole_ver
+	local extra_paholeopt=
 
 	if ! [ -x "$(command -v ${PAHOLE})" ]; then
 		echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available"
@@ -227,8 +228,12 @@ gen_btf()
 
 	vmlinux_link ${1}
 
+	if [ "${pahole_ver}" -ge "121" ]; then
+		extra_paholeopt="${extra_paholeopt} --btf_gen_floats"
+	fi
+
 	info "BTF" ${2}
-	LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
+	LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${extra_paholeopt} ${1}
 
 	# Create ${2} which contains just .BTF section but no symbols. Add
 	# SHF_ALLOC because .BTF will be part of the vmlinux image. --strip-all