runtime: in __tsan_fini tell scheduler we are entering non-Go code

runtime: in __tsan_fini tell scheduler we are entering non-Go code

__tsan_fini will call exit which will call destructors which
may in principle call back into Go functions. Prepare the scheduler
by calling entersyscall before __tsan_fini.

Fixes #59711

Change-Id: Ic4df8fba3014bafa516739408ccfc30aba4f22ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/486615
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>