Make toolchain more compose-y

Remove many of the overridden methods from toolchainBase in favor
of having smaller composable types.  toolchainNoCrt, toolchain32Bit
and toolchain64Bit now only provide a few methods and can be mixed
in to any toolchain type.  toolchainLinux, toolchainBionic,
toolchainWindows and toolchainDarwin now embed toolchainBase to provide
the default flags methods for when they don't override them.

This avoids the need for disambiguation methods required when a
type embeds two types that implement the same method.

Test: cc_test.go
Change-Id: I641da2a47aba597c517f693efedb65cf41273c82