[libc++] Avoid relying on non-portable behaviour in std::align

Round-tripping pointers via size_t is not portable, the C/C++ standards
only require this to be valid when using (u)intptr_t.
Originally committed to the CHERI fork of LLVM as
dd01245185,
but I forgot to upstream the change. I rediscovered this issue due to a
compiler warning when building libc++ on a Arm Morello system.

Reviewed By: #libc, ldionne, philnik

Differential Revision: https://reviews.llvm.org/D134363

libcxx/src/memory.cpp

@@ -194,7 +194,7 @@ align(size_t alignment, size_t size, void*& ptr, size_t& space)
     if (size <= space)
     {
         char* p1 = static_cast<char*>(ptr);
-        char* p2 = reinterpret_cast<char*>(reinterpret_cast<size_t>(p1 + (alignment - 1)) & -alignment);
+        char* p2 = reinterpret_cast<char*>(reinterpret_cast<uintptr_t>(p1 + (alignment - 1)) & -alignment);
         size_t d = static_cast<size_t>(p2 - p1);
         if (d <= space - size)
         {