Solarish: standard allocator uses `memalign` incorrectly

[RalfJung]

The docs for that function state

The memalign() function allocates size bytes on a specified alignment boundary and returns a pointer to the allocated block. The value of the returned address is guaranteed to be an even multiple of alignment. The value of alignment must be a power of two and must be greater than or equal to the size of a word.

(emphasis mine)

However, the way this function gets called here does not ensure that the alignment is at least the size of a word.

Cc @devnexen (discovered by failing CI in rust-lang/miri#3575)