cmd/internal/obj/ppc64: combine C_SBRA and C_LBRA optab classes

cmd/internal/obj/ppc64: combine C_SBRA and C_LBRA optab classes

The assembler treats C_SBRA and C_LBRA optab classes identically,
combine them into one class to reduce the number of optab classes.

Likewise, C_LBRAPIC is renamed to C_BRAPIC for consistency with
the above change.

Change-Id: I47000e7273cb8f89a4d0621d71433ccbfb7afb70
Reviewed-on: https://go-review.googlesource.com/c/go/+/557916
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: David Chase <drchase@google.com>