Revert "runtime/cgo: use pthread_attr_get_np on Illumos"

This reverts CL 481795.

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: I5ec1f495154205ebdf19cd44c6e6452a7a3606f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/485315
Auto-Submit: Michael Pratt <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Michael Pratt <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>

Author: prattmic

src/runtime/cgo/gcc_stack_unix.c

@@ -18,16 +18,9 @@ x_cgo_getstackbound(G *g)
 	void *addr;
 	size_t size;
 
-#if defined(__GLIBC__) || (defined(__sun) && !defined(__illumos__))
-	// pthread_getattr_np is a GNU extension supported in glibc.
-	// Solaris is not glibc but does support pthread_getattr_np
-	// (and the fallback doesn't work...). Illumos does not.
+#if defined(__GLIBC__) || defined(__sun)
 	pthread_getattr_np(pthread_self(), &attr);  // GNU extension
 	pthread_attr_getstack(&attr, &addr, &size); // low address
-#elif defined(__illumos__)
-	pthread_attr_init(&attr);
-	pthread_attr_get_np(pthread_self(), &attr);
-	pthread_attr_getstack(&attr, &addr, &size); // low address
 #else
 	pthread_attr_init(&attr);
 	pthread_attr_getstacksize(&attr, &size);