Fix incremental builds: rm before creating .a archives
Fix incremental builds: rm before creating .a archives

Turns out that 'ar rcsD' appends entries to the .a and doesn't
remove old entries. This creates linker errors when doing incremental
builds across a significant build file change (which go away by doing
gn clean).

Test: manual
Change-Id: I358976db180b03ad073f5bf2ec13b6217e7fefb8

google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto