[libc++] Use aligned_alloc instead of posix_memalign for C++17

C++17 defines the C11 `aligned_alloc`, so we can use that instead of
posix_memalign. This change allows building against picolibc without
defining _DEFAULT_SOURCE/_GNU_SOURCE.
The C11 `aligned_alloc` function should be available on all supported
non-Windows platforms except for macOS where we need version 10.15.

There is one caveat: aligned_alloc() requires that __size is a multiple of
__alignment, but [new.delete.general] only states "if the value of an
alignment argument passed to any of these functions is not a valid
alignment value, the behavior is undefined".
To handle calls such as ::operator new(1, std::align_val_t(128)), we
round up __size to __alignment (and check for wrap-around).
This is required at least for macOS where aligned_alloc(128, 1) returns
an error instead of allocating memory (glibc ignores the specification).

Differential Revision: https://reviews.llvm.org/D138196