pw_toolchain: Use build args when skipping static analysis

This CL reworks the way in which Pigweed-supported (but not
Pigweed-provided) third party code is excluded from static analysis.
Rather than specifying regular expressions for source paths and include
paths in the top-level .gn file, it uses the values of specific
`dir_pw_third_party_*` build args when set.

The `pw_toolchain_SKIP_STATIC_ANALYSIS_*` build args are preserved, and
appended to the values generated from the third party build args. this
allows greater flexibility, as third party repos can reside anywhere
locally and downstream consumers do not need to et special values when
using Pigweed-supported third party code.

Change-Id: I16cecb52e216ca5f8ec6de673caec43d84897ce7
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/140114
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Aaron Green <aarongreen@google.com>