[libc] Use global errno for baremetal

We want to avoid thread local variables on baremetal since the TLS
support may not be universally available in those environments.

Author: petrhosek

libc/include/errno.h.def

@@ -25,7 +25,8 @@
 #include "llvm-libc-macros/generic-error-number-macros.h"
 #endif
 
-#if defined(__AMDGPU__) || defined(__NVPTX__)
+#if defined(__AMDGPU__) || defined(__NVPTX__) || \
+  (defined(__ELF__) && !defined(__linux__) && defined(__Fuchsia__))
 extern int __llvmlibc_errno; // Not thread_local!
 #else
 #ifdef __cplusplus

libc/src/errno/libc_errno.cpp

@@ -9,7 +9,8 @@
 #include "libc_errno.h"
 #include "src/__support/CPP/atomic.h"
 
-#ifdef LIBC_TARGET_ARCH_IS_GPU
+#if defined(LIBC_TARGET_ARCH_IS_GPU) || \
+  (defined(__ELF__) && !defined(__linux__) && defined(__Fuchsia__))
 // LIBC_THREAD_LOCAL on GPU currently does nothing. So essentially this is just
 // a global errno for gpu to use for now.
 extern "C" {