bpf: TARGET_INSN_COST: don't hardcode 1 for default ...

... instead use pattern_cost () as used by other backends

gcc/ChangeLog:

	* config/bpf/bpf.cc (bpf_insn_cost): Return pattern_cost () by default.

Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
(cherry picked from commit 3c71ff89bb03152828f0efc5f1de5febf35fc039)