[libc] Implement 'errno' on the GPU as a global integer internally

[jhuber6]

Summary

The C standard asserts that the errno value is an l-value thread local
integer. We cannot provide a generic thread local integer on the GPU
currently without some workarounds. Previously, we worked around this by
implementing the errno value as a special consumer class that made all
the writes disappear. However, this is problematic for internal tests.
Currently there are build failures because of this handling and it's
only likely to cause more problems the more we do this.

This patch instead makes the internal target used for testing export the
errno value as a simple global integer. This allows us to use and test
the errno interface correctly assuming we run with a single thread.
Because this is only used for the non-exported target we still do not
provide this feature in the version that users will use so we do not
need to worrk about it being incorrect in general.

libc/src/errno/libc_errno.h

 // This macro will resolve to errno from the errno.h file included above. Under
 // full build, this will be LLVM libc's errno. In overlay build, it will be
 // system libc's errno.
 #define libc_errno errno
 #endif
 #else
 namespace __llvm_libc {
 
-#ifdef LIBC_TARGET_ARCH_IS_GPU
-extern "C" ErrnoConsumer __llvmlibc_internal_errno;
-#else // LIBC_TARGET_ARCH_IS_GPU
-extern "C" {
-extern LIBC_THREAD_LOCAL int __llvmlibc_internal_errno;
-} // extern "C"
-#endif
+// TODO: On the GPU build this will be mapped to a single global value. We need
+// to ensure that tests are not run with multiple threads that depend on errno
+// until we have true 'thread_local' support on the GPU.
+extern "C" LIBC_THREAD_LOCAL int __llvmlibc_internal_errno;
 
 // TODO: After all of libc/src and libc/test are switched over to use
 // libc_errno, this header file will be "shipped" via an add_entrypoint_object
 // target. At which point libc_errno, should point to __llvmlibc_internal_errno
 // if LIBC_COPT_PUBLIC_PACKAGING is not defined.
 #define libc_errno __llvm_libc::__llvmlibc_internal_errno
 
 } // namespace __llvm_libc
 #endif
 
 #endif // LLVM_LIBC_SRC_ERRNO_LLVMLIBC_ERRNO_H

libc/src/errno/libc_errno.cpp

 // TODO: Use a macro to distinguish full build and overlay build which can be
 //       used to exclude __llvmlibc_errno under overlay build.
 #ifdef LIBC_TARGET_ARCH_IS_GPU
 ErrnoConsumer __llvmlibc_errno;
 #else
 LIBC_THREAD_LOCAL int __llvmlibc_errno;
 #endif // LIBC_TARGET_ARCH_IS_GPU
 #else
-#ifdef LIBC_TARGET_ARCH_IS_GPU
-ErrnoConsumer __llvmlibc_internal_errno;
-#else
 LIBC_THREAD_LOCAL int __llvmlibc_internal_errno;
-#endif // LIBC_TARGET_ARCH_IS_GPU
 #endif
 } // extern "C"
 
 } // namespace __llvm_libc