remove the use of _LIBCXXABI_HIDDEN

Summary:
The macro `_LIBCXXABI_HIDDEN` is seeminly unavailable on FreeBSD so avoid its use in the exception-ptr abi accessors.

Fixes: https://github.com/facebook/folly/issues/1570.

Reviewed By: luciang

Differential Revision: D28217011

fbshipit-source-id: c745ae468c840c33df921c68fb06759c078b6df0

folly/lang/Exception.cpp

@@ -50,7 +50,7 @@
 
 namespace __cxxabiv1 {
 
-struct _LIBCXXABI_HIDDEN __cxa_exception {
+struct __cxa_exception {
 #if defined(__LP64__) || defined(_WIN64) || defined(_LIBCXXABI_ARM_EHABI)
   void* reserve;
   size_t referenceCount;