Add Toolchain.Bionic()

Move some cc ctx.Host() / ctx.Device() checks over to using
ctx.toolchain().Bionic(). There will be more changes, these are just the
obvious ones dealing with host_ldlibs / crt / system libraries.

Bug: 31559095
Test: out/soong/build.ninja is identical
Change-Id: Ibba60483b4ab0e01f6996eb7d883120e4acc1830

cc/config/toolchain.go

@@ -74,6 +74,8 @@ type Toolchain interface {
 	SanitizerRuntimeLibraryArch() string
 
 	AvailableLibraries() []string
+
+	Bionic() bool
 }
 
 type toolchainBase struct {
@@ -133,6 +135,10 @@ func (toolchainBase) AvailableLibraries() []string {
 	return []string{}
 }
 
+func (toolchainBase) Bionic() bool {
+	return true
+}
+
 type toolchain64Bit struct {
 	toolchainBase
 }