[libc] Mark internal __llvm_libc_errno as noexcept (llvm#98760)

The declaration must match the previous declaration in errno.h.

Author: petrhosek

libc/src/errno/libc_errno.h

@@ -33,7 +33,7 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
-extern "C" int *__llvm_libc_errno();
+extern "C" int *__llvm_libc_errno() noexcept;
 
 struct Errno {
   void operator=(int);