use standard C11 alignas

use standard C11 alignas

nnn source code already uses C11, which introduced `_Alignas` [^0] as a
standardized alternative to __attribute__((aligned)).

there are no other usage of `__attribute__` in the nnn source code, so
in theory this should make things more portable across compilers.

also removes an unnecessary usage of `alloca()` with an aligned constant
sized buffer instead.

[^0]: https://en.cppreference.com/w/c/language/_Alignas