bpf: generate indirect calls for xBPF

bpf: generate indirect calls for xBPF

This patch updates the BPF back end to generate indirect calls via
the 'call %reg' instruction when targetting xBPF.

Additionally, the BPF ASM_SPEC is updated to pass along -mxbpf to
gas, where it is now supported.

2020-09-03  David Faust  <david.faust@oracle.com>

gcc/

	* config/bpf/bpf.h (ASM_SPEC): Pass -mxbpf to gas, if specified.
	* config/bpf/bpf.c (bpf_output_call): Support indirect calls in xBPF.

gcc/testsuite/

	* gcc.target/bpf/xbpf-indirect-call-1.c: New test.