cpu,opcodes,gas: use %r0 and %r6 instead of %a and %ctf in eBPF disassembler

cpu,opcodes,gas: use %r0 and %r6 instead of %a and %ctf in eBPF disassembler

This patch changes the eBPF CPU description to prefer the register
names %r0 and %r6 instead of %a and %ctx when disassembling.  This
matches better with the current practice, vs. cBPF.

It also updates the GAS tests in order to reflect this change.
Tested in a x86_64 host.

cpu/ChangeLog:

2019-07-19  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* bpf.cpu (h-gpr): when disassembling, use %r0 and %r6 instead of
	%a and %ctx.

opcodes/ChangeLog:

2019-07-19  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* bpf-desc.c: Regenerated.

gas/ChangeLog:

2019-07-19  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* testsuite/gas/bpf/alu.d: Use %r6 instead of %ctx.
	* testsuite/gas/bpf/lddw-be.d: Likewise.
	* testsuite/gas/bpf/lddw.d: Likewise.
	* testsuite/gas/bpf/alu-be.d: Likewise.
	* testsuite/gas/bpf/alu32.d: Likewise.