Update compdb to put full path to compiler

Some language servers (vscode) seem to have started to rely on the
compiler being an absolute path recently. This updates the compdb
generator to match this expectation.

Bug: https://github.com/Microsoft/vscode-cpptools/issues/3112
Test: m nothing && m SOONG_GEN_COMPDB=1 SOONG_LINK_COMPDB_TO=$ANDROID_BUILD_TOP nothing
Test: Open vscode
Change-Id: Id146d69c2b808b23896132bbd4bc38ed5c7fd9c7