[gcc r11-8275] [libstdc++] Add missing _M_try_acquire() to __platform_semaphore

[Thomas Rodgers]

https://gcc.gnu.org/g:e826f7355af74d3e0be5ceee779c74eca7ddbfc4

commit r11-8275-ge826f7355af74d3e0be5ceee779c74eca7ddbfc4
Author: Thomas Rodgers <rodgert@twrodgers.com>
Date:   Wed Apr 21 08:15:16 2021 -0700

    [libstdc++] Add missing _M_try_acquire() to __platform_semaphore
    
    libstdc++-v3/ChangeLog:
            * include/bits/semaphore_base.h: Add missing _M_try_acquire()
            member to __platform_wait.
    
    (cherry picked from commit 5445da1a94bb4df752209e54f4aa21702609a20a)

Diff:
---
 libstdc++-v3/include/bits/semaphore_base.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/libstdc++-v3/include/bits/semaphore_base.h b/libstdc++-v3/include/bits/semaphore_base.h
index 7e3235d182e..35469e443b0 100644
--- a/libstdc++-v3/include/bits/semaphore_base.h
+++ b/libstdc++-v3/include/bits/semaphore_base.h
@@ -86,6 +86,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	}
     }
 
+    _GLIBCXX_ALWAYS_INLINE bool
+    _M_try_acquire() noexcept
+    {
+      for (;;)
+	{
+	  auto __err = sem_trywait(&_M_semaphore);
+	  if (__err && (errno == EINTR))
+	    continue;
+	  else if (__err && (errno == EAGAIN))
+	    return false;
+	  else if (__err)
+	    std::terminate();
+	  else
+	    break;
+	}
+      return true;
+    }
+
     _GLIBCXX_ALWAYS_INLINE void
     _M_release(std::ptrdiff_t __update) noexcept
     {