[libc] Partially implement 'errno' on the GPU

Summary:
The `errno` variable is expected to be `thread_local` by the standard.
However, the GPU targets do not support `thread_local` and implementing
that would be a large endeavor. Because of that, we previously didn't
provide the `errno` symbol at all. However, to build some programs we at
least need to be able to link against `errno`. Many things that would
normally set `errno` completely ignore it currently (i.e. stdio) but
some programs still need to be able to link against correct C programs.

For this purpose this patch exports the `errno` symbol as a simple
global. Internally, this will be updated atomically so it's at least not
racy. Externally, this will be on the user. I've updated the
documentation to state as such. This is required to get `libc++` to
build.

Author: jhuber6

libc/include/errno.h.def

@@ -25,17 +25,18 @@
 #include "llvm-libc-macros/generic-error-number-macros.h"
 #endif
 
-#if !defined(__AMDGPU__) && !defined(__NVPTX__)
-
+#if defined(__AMDGPU__) || defined(__NVPTX__)
+extern int __llvmlibc_errno; // Not thread_local!
+#else
 #ifdef __cplusplus
 extern "C" {
 extern thread_local int __llvmlibc_errno;
 }
 #else
 extern _Thread_local int __llvmlibc_errno;
 #endif // __cplusplus
+#endif
 
 #define errno __llvmlibc_errno
-#endif
 
 #endif // LLVM_LIBC_ERRNO_H

libc/src/errno/libc_errno.cpp

@@ -7,16 +7,21 @@
 //===----------------------------------------------------------------------===//
 
 #include "libc_errno.h"
+#include "src/__support/CPP/atomic.h"
 
 #ifdef LIBC_TARGET_ARCH_IS_GPU
-// LIBC_THREAD_LOCAL on GPU currently does nothing.  So essentially this is just
+// LIBC_THREAD_LOCAL on GPU currently does nothing. So essentially this is just
 // a global errno for gpu to use for now.
 extern "C" {
-LIBC_THREAD_LOCAL int __llvmlibc_gpu_errno;
+LIBC_THREAD_LOCAL LIBC_NAMESPACE::cpp::Atomic<int> __llvmlibc_errno;
 }
 
-void LIBC_NAMESPACE::Errno::operator=(int a) { __llvmlibc_gpu_errno = a; }
-LIBC_NAMESPACE::Errno::operator int() { return __llvmlibc_gpu_errno; }
+void LIBC_NAMESPACE::Errno::operator=(int a) {
+  __llvmlibc_errno.store(a, cpp::MemoryOrder::RELAXED);
+}
+LIBC_NAMESPACE::Errno::operator int() {
+  return __llvmlibc_errno.load(cpp::MemoryOrder::RELAXED);
+}
 
 #elif !defined(LIBC_COPT_PUBLIC_PACKAGING)
 // This mode is for unit testing.  We just use our internal errno.