Stop using expressions in _Alignof()

_Alignof(expression) is a non-standard extension. This is not allowed
in gnu11 and gnu17 which follow the C11 standard _Alignof(type).

Reviewed by: arichardson, imp, jhb
Fixes: 4a9cd9fc22d7 amd64 db_trace: Reject unaligned frame pointers
Fixes: 7ccaf76a27f1 riscv db_trace: Ensure trapframe pointer is suitably aligned.
Fixes: 638c68897fbd arm64 db_trace: Ensure trapframe pointer is suitably aligned.
Differential Revision: https://reviews.freebsd.org/D43409

(cherry picked from commit 03d04bf49492fc70366e6d78194336a4122282a2)