[PA] Don't use defined() in PA_CONFIG_* macros

This will trigger -Wno-expansion-to-defined, which is used by future
PA library users, like dawn.

This is a pure refactor, no codegen changes.

Bug: 1498930
Change-Id: Ifad53534e490b1b00338a9f5ce4505f09eb3ac28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5001451
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1220178}


CrOS-Libchrome-Original-Commit: 1009ecaa0ac20f4aa44d7e681b24200f4ff443f9