This adds the support to enable the race detector for ppc64le.

Added runtime/race_ppc64le.s to manage the calls from Go to the
LLVM tsan functions, mostly converting from the Go ABI to the
PPC64 ABI expected by Clang generated code.

Changed racewalk.go to call racefuncenterfp instead of racefuncenter
on ppc64le to allow the caller pc to be obtained in the asm code
before calling the tsan version.

Changed the set up code for racecallbackthunk so it doesn't use
the autogenerated save and restore of the link register since that
sequence uses registers inconsistent with the normal ppc64 ABI.

Made various changes to recognize that race is supported for
ppc64le.

Ensured that tls_g is updated and accessible from race_linux_ppc64le.s
so that the race ctx can be obtained and passed to tsan functions.

This enables the race tests for ppc64le in cmd/dist/test.go and
increases the timeout when running the benchmarks with the -race
option to avoid timing out.

Updates #24354, #23731

Change-Id: Ib97dc7ac313e6313c836dc7d2fb698f9d8fba3ef
Reviewed-on: https://go-review.googlesource.com/107935
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>