bpf: use the expected instruction for NOPs

bpf: use the expected instruction for NOPs

The BPF ISA doesn't have a no-operation instruction, but in practice
the Linux kernel verifier performs some optimizations that rely on
these instructions to be encoded in a particular way.  As it turns
out, we were using the "wrong" instruction in GCC.

This patch makes GCC to generate the expected instruction for NOP (a
`ja 0') and also adds a test to make sure this is the case.

Tested in bpf-unknown-none targets.

2020-09-14  Jose E. Marchesi  <jose.marchesi@oracle.com>

gcc/

	* config/bpf/bpf.md ("nop"): Re-define as `ja 0'.

gcc/testsuite/

	* gcc.target/bpf/nop-1.c: New test.