cg: wrap dctx and dctx->member accesses in a function

Introduce dt_cg_access_dctx() to implement access to the DTrace context
and members therein for both regular programs and trampolines.

Replace
	BPF_LOAD(BPF_DW, <reg>, BPF_REG_FP, DT_STK_DCTX)
	BPF_LOAD(<size>, <reg>, <reg>, <member>)
with
	dt_cg_access_dctx(<reg>, dlp, drp, <member>)

Replace
	BPF_LOAD(BPF_DW, <reg>, BPF_REG_FP, DT_STK_DCTX)
	(not followed by access to a member of dctx)
with
	dt_cg_access_dctx(<reg>, dlp, drp, -1)

Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Reviewed-by: Nick Alcock <nick.alcock@oracle.com>