Some tests in misc/cgo/test are run with various options including
'-linkmode=external "-extldflags=-pie"'. On ppc64x passing -pie to
the external linker with code that was not compiled as position
independent is incorrect. This works by luck in many cases but is
not guaranteed to work. I suspect it is an issue on other targets
as well.

This will now run the tests using -buildmode=pie for the platforms
that support that buildmode option.

Fixes #21954

Change-Id: I25fc7573f2d3cb5b0d1c691a0ac91aef7715404f
Reviewed-on: https://go-review.googlesource.com/66870
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>