tools/runqslower: Build bpftool using HOSTCC

When cross building runqslower for an other architecture, the
intermediate bpftool used to generate a skeleton must be built using the
host toolchain. Pass HOSTCC and HOSTLD, defined in Makefile.include, to
the bpftool Makefile.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201110164310.2600671-7-jean-philippe@linaro.org