Add DefaultSharedLibraries to Toolchain

Move the default value for system_shared_libs out of linker.go and into
a new DefaultSharedLibraries method in Toolchain.  This will simplify
adding system_shared_libs to cc_object, and adding new toolchains with
different default system_shared_libs.

Test: m checkbuild
Test: go test ./cc/...
Change-Id: I422315f090be20719c08a4d33bd39b0b26a93d6c