[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
https://github.com/CTSRD-CHERI/llvm-project/commit/dd01245185ab9e71b70b418bee8f11ea0199e1a3,
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