tools, bpf: Do not force gcc as CC

This allows transparent cross-compilation with CROSS_COMPILE by
relying on 7ed1c190 ("tools: fix cross-compile var clobbering").

Same change was applied to tools/bpf/bpftool/Makefile in
9e88b931 ("tools: bpftool: do not force gcc as CC").

Signed-off-by: Brett Mastbergen <brett.mastbergen@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200609213506.3299-1-brett.mastbergen@gmail.com