Teach libc++ to use native NetBSD's max_align_t

Summary:
The NetBSD headers ship with max_align_t, that is not
compatible with the fallback version in libc++.

There is no defined a compiler specific symbol in the headers like:
 - __CLANG_MAX_ALIGN_T_DEFINED
 - _GCC_MAX_ALIGN_T
 - __DEFINED_max_align_t

Sponsored by <The NetBSD Foundation>

Reviewers: chandlerc, dlj, EricWF, joerg

Reviewed By: joerg

Subscribers: bsdjhb, llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D47814

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@340224 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/cstddef b/include/cstddef
index 1dc5d39..b45dbe2 100644
--- a/include/cstddef
+++ b/include/cstddef
@@ -50,7 +50,7 @@
 using ::size_t;
 
 #if defined(__CLANG_MAX_ALIGN_T_DEFINED) || defined(_GCC_MAX_ALIGN_T) || \
-    defined(__DEFINED_max_align_t)
+    defined(__DEFINED_max_align_t) || defined(__NetBSD__)
 // Re-use the compiler's <stddef.h> max_align_t where possible.
 using ::max_align_t;
 #else
diff --git a/include/stddef.h b/include/stddef.h
index faf8552..f65065d 100644
--- a/include/stddef.h
+++ b/include/stddef.h
@@ -54,7 +54,7 @@
 
 // Re-use the compiler's <stddef.h> max_align_t where possible.
 #if !defined(__CLANG_MAX_ALIGN_T_DEFINED) && !defined(_GCC_MAX_ALIGN_T) && \
-    !defined(__DEFINED_max_align_t)
+    !defined(__DEFINED_max_align_t) && !defined(__NetBSD__)
 typedef long double max_align_t;
 #endif