bpf: use xBPF signed div, mod insns when available

bpf: use xBPF signed div, mod insns when available

The 'mod' and 'div' operators in eBPF are unsigned, with no signed
counterpart. xBPF adds two new ALU operations, sdiv and smod, for
signed division and modulus, respectively. Update bpf.md with
'define_insn' blocks for signed div and mod to use them when targetting
xBPF, and add new tests to ensure they are used appropriately.

2020-09-17  David Faust  <david.faust@oracle.com>

gcc/
	* config/bpf/bpf.md: Add defines for signed div and mod operators.

gcc/testsuite/
	* gcc.target/bpf/diag-sdiv.c: New test.
	* gcc.target/bpf/diag-smod.c: New test.
	* gcc.target/bpf/xbpf-sdiv-1.c: New test.
	* gcc.target/bpf/xbpf-smod-1.c: New test.