runtime: use -buildmode=pie in testCgoPprofPIE instead of -extldflags=-pie

Errors occur in runtime test testCgoPprofPIE when the test
is built by passing -pie to the external linker with code
that was not built as PIC. This occurs on ppc64le because
non-PIC is the default, and fails only on newer distros
where the address range used for programs is high enough
to cause relocation overflow. This test should be built
with -buildmode=pie since that correctly generates PIC
with -pie.

Related issues are #21954 and #22126.

Updates #22459

Change-Id: Ib641440bc9f94ad2b97efcda14a4b482647be8f7
Reviewed-on: https://go-review.googlesource.com/73970
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/crash_cgo_test.go

@@ -330,7 +330,7 @@ func TestCgoPprof(t *testing.T) {
 }
 
 func TestCgoPprofPIE(t *testing.T) {
-	testCgoPprof(t, "-ldflags=-extldflags=-pie", "CgoPprof")
+	testCgoPprof(t, "-buildmode=pie", "CgoPprof")
 }
 
 func TestCgoPprofThread(t *testing.T) {