bp2build: export some cc toolchain flags into Starlark.

This CL exports common/global/device/host clang/ld/ldd flags
from their Ninja variable initialization locations in
cc/config/global.go and cc/config/clang.go to make Bazel's cc_toolchain
and Soong's cc actions more consistent with each other.

This does not handle env-dependent or arch-specific toolchain flags
yet (logic in compiler.go and linker.go).

Test: TH
Bug: 187086342
Bug: 187084737
Bug: 186628704
Bug: 187857770
Change-Id: Ie403d7cd23f35160897b9dd902c799cbf1bd7f0c