On ppc64x, trampolines are used to resolve too-far
branches for internal linking.  The external linking,
solution on ppc64x is to split text sections when they
get too large, allowing the external linker to handle
the long branches.

On arm trampolines are generanted for too-far branches
for internal and external linking.  When the change
was made recently to enable trampolines for external linking
on arm, that broke the ppc64x fix for too-far branches
with external linking.

The fix adds a check to use trampolines only for internal
linking with ppc64x.

Fixes #17795

Change-Id: Icce968fb96545f10a913e07654514643bce96261
Reviewed-on: https://go-review.googlesource.com/32853
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>