Rename Target to Arch in header-checker utils.py

"target" refers to a target arch, a build target, or a make target.
This commit renames the class and the variables to avoid ambiguity.

Test: development/vndk/tools/header-checker/utils/create_reference_dumps.py \
      -products aosp_x86_64 -release trunk_staging
Bug: 297542516
Change-Id: Icfb06e18333d56af802c74494c1d158f367026c9
2 files changed