floss: Make __file__ relative to root

Update GN rules so that __file__ is always relative to
bt/system/BUILD.gn rather than the GN root build directory.

Bug: 210526127
Tag: #floss
Test: Observe logs on ChromeOS has shorter __file__ paths
Change-Id: Ia253a75fa18f5f0d2785f2e6b5f69e02890ad294