Stop using expressions in _Alignof()

[minsoochoo0122_proton.me]

Summary

Divide from D43239

_Alignof(expression) is a non-standard extension (https://en.cppreference.com/w/c/language/_Alignof).
This is not allowed in gnu11 and gnu17 which follow the C11 standard _Alignof(type).

Test Plan

make buildkernel

[jrtc27]

Diff itself is fine, but the amended summary isn't a helpful commit message

[imp]

Please wrap the commit message at 80 columns. Otherwise it looks much better.

[jrtc27]

Sorry, just spotted one style issue

sys/amd64/amd64/db_trace.c

208–209

This line is now too long

[imp]

Oh how I wish I could run even the kinda whiny checkstyle9.pl in phabricator :(.

[minsoochoo0122_proton.me]

style(9)

[minsoochoo0122_proton.me]

Fix commit message

[jhb]

I will add a Fixes: tag to reference my original commit to the log message when merging this. Also, no need to mention the other review (first sentence) in the commit log so I will elide that as well.