Use sched_yield instead of pthread_yield

pthread_yield is not a standard POSIX function, and is not available
in musl libc. The man page says to "Use the standardized sched_yield(2)
instead"[0].

On glibc, pthread_yield is exactly equivalent to sched_yield[1].

On bionic, pthread_yield is also not available, so on Android, the
tests define a wrapper that just calls sched_yield. This wrapper
is no longer necessary if we just use sched_yield in the first
place.

[0] http://man7.org/linux/man-pages/man3/pthread_yield.3.html
[1] https://sourceware.org/git/?p=glibc.git;a=blob;f=nptl/pthread_yield.c

Bug: google-breakpad:631
Change-Id: Ie4c6be8c17cdc2f5396a7fe972fa51a97573b049
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/2097340
Reviewed-by: Mike Frysinger <vapier@chromium.org>

src/common/android/testing/pthread_fixes.h

diff --git a/src/common/android/testing/pthread_fixes.h b/src/common/android/testing/pthread_fixes.h
index 15c6309..20c1208 100644
--- a/src/common/android/testing/pthread_fixes.h
+++ b/src/common/android/testing/pthread_fixes.h
@@ -89,11 +89,6 @@
 
 #endif  // defined(PTHREAD_BARRIER_SERIAL_THREAD)
 
-int pthread_yield(void) {
-  sched_yield();
-  return 0;
-}
-
 }  // namespace
 
 #endif  // GOOGLE_BREAKPAD_COMMON_ANDROID_TESTING_PTHREAD_FIXES_H

src/common/linux/tests/crash_generator.cc

diff --git a/src/common/linux/tests/crash_generator.cc b/src/common/linux/tests/crash_generator.cc
index 3631967..6896a68 100644
--- a/src/common/linux/tests/crash_generator.cc
+++ b/src/common/linux/tests/crash_generator.cc
@@ -33,6 +33,7 @@
 #include "common/linux/tests/crash_generator.h"
 
 #include <pthread.h>
+#include <sched.h>
 #include <signal.h>
 #include <stdio.h>
 #include <sys/mman.h>
@@ -88,7 +89,7 @@
     exit(1);
   }
   while (true) {
-    pthread_yield();
+    sched_yield();
   }
 }