If the host has an SV39 MMU, the usual aligned hinting strategy will
not work despite the system being 64-bit. On RISC-V linux systems, the
type of MMU can be read from /proc/cpuinfo. If we find one, a constant
is defined that will allow us to skip the aligned hinting when the
time comes.

This constant will be defined when the host has an SV39 MMU. The usual
2TiB+ hinting strategy will not work on those systems, so we skip it,
just as we would if the system was not "64-bit."

On 32-bit or SV39 systems, _mi_os_get_aligned_hint() in src/os.c is
ifdef'd to always return NULL. As a result, the OS prim allocation
routines using that hint will fail to obtain aligned memory. When that
happens, we free() the result, and overallocate.

But this is wasteful: we know that (with high probability) the first
attempt at an aligned allocation will fail. This commit modifies
mi_os_prim_alloc_aligned() to skip the doomed alloc and subsequent
free using the same ifdef condition that is used to disable the hint.