Revert "Revert "Revert "runtime: consolidate function descriptor definitions on PPC64"""

Revert "Revert "Revert "runtime: consolidate function descriptor definitions on PPC64"""

This reverts CL 481075 (a re-apply of previously reverted CL 478917).

Reason for revert: CL 481061 causes C TSAN failures and must be
reverted. See CL 485275. This CL depends on CL 481061.

For #59678.

Change-Id: I4bf7f43d9df1ae28e04cd4065552bcbee82ef13f
Reviewed-on: https://go-review.googlesource.com/c/go/+/485316
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>