bpf: add -mcpu and related feature options

bpf: add -mcpu and related feature options

New instructions have been added over time to the eBPF ISA, but
previously there has been no good method to select which version to
target in GCC.

This patch adds the following options to the BPF backend:

  -mcpu={v1, v2, v3}
    Select which version of the eBPF ISA to target. This enables or
    disables generation of certain instructions. The default is v3.

  -mjmpext
    Enable extra conditional branch instructions.
    Enabled for CPU v2 and above.

  -mjmp32
    Enable 32-bit jump/branch instructions.
    Enabled for CPU v3 and above.

  -malu32
    Enable 32-bit ALU instructions.
    Enabled for CPU v3 and above.

gcc/ChangeLog:
	* config/bpf/bpf-opts.h (bpf_isa_version): New enum.
	* config/bpf/bpf-protos.h (bpf_expand_cbranch): New.
	* config/bpf/bpf.c (bpf_option_override): Handle -mcpu option.
	(bpf_expand_cbranch): New function.
	* config/bpf/bpf.md (AM mode iterator): Conditionalize support for SI
	mode.
	(zero_extendsidi2): Only use mov32 instruction if it is available.
	(SIM mode iterator): Conditionalize support for SI mode.
	(JM mode iterator): New.
	(cbranchdi4): Update name, use new JM iterator. Use bpf_expand_cbranch.
	(*branch_on_di): Update name, use new JM iterator.
	* config/bpf/bpf.opt: (mjmpext): New option.
	(malu32): Likewise.
	(mjmp32): Likewise.
	(mcpu): Likewise.
	(bpf_isa): New enum.