bp2build: Handle export_generated_header property

The generated_header property resides in BaseCompilerProperties, while
export_generated_header resides in BaseLinkerProperties. Previously
bp2build handled these property structs separately; however, these two
related properties residing in separate structs requires restructuring
the code to allow access to both BaseCompilerProperties and
BaseLinkerProperties for a single axis/configuration combination to
resolve which generated headers are exported/not for the
axis/configuration..

Test: go test soong tests
Test: build/bazel/ci/bp2build.sh
Change-Id: Id150003637fd19d87e8dc5d6941e9f36dc4031dd