Add default_shared_libs property

Building against a locally-built libc besides bionic requires setting
system_shared_libs to avoid circular dependencies, but modules all
over the tree assume that system_shared_libs only affects modules
building against bionic.  Add a new default_shared_libs property
that applies to all modules, which will generally be set in
arch-specific clauses to only affect the desired set of modules.

Bug: 190084016
Test: m checkbuild
Change-Id: Ia2349d84c70e503916f90a5d2702e135248f73df
diff --git a/cc/cc.go b/cc/cc.go
index 0c9f945..d11357f 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -571,7 +571,8 @@
 	sharedLibs []string
 	// Note nil and [] are semantically distinct. [] prevents linking against the defaults (usually
 	// libc, libm, etc.)
-	systemSharedLibs []string
+	systemSharedLibs  []string
+	defaultSharedLibs []string
 }
 
 // installer is the interface for an installer helper object. This helper is responsible for