btrace(8): add cpu to long conversion

[Dave Voutila]

Pretty simple...allow using the "cpu" special variable where a long is
expected, like in hist/lhist.

(note: the switch/case is in enum order, not alphabetical order)

OK?

Index: btrace.c
===================================================================
RCS file: /cvs/src/usr.sbin/btrace/btrace.c,v
retrieving revision 1.35
diff -u -p -r1.35 btrace.c
--- btrace.c    22 Apr 2021 09:36:39 -0000      1.35
+++ btrace.c    4 Jun 2021 12:47:27 -0000
@@ -1180,6 +1180,9 @@ ba2long(struct bt_arg *ba, struct dt_evt
        case B_AT_BI_TID:
                val = dtev->dtev_tid;
                break;
+       case B_AT_BI_CPU:
+               val = dtev->dtev_cpu;
+               break;
        case B_AT_BI_NSECS:
                val = builtin_nsecs(dtev);
                break;