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

Android.bp

@@ -209,6 +209,7 @@ cc_defaults {
 
 toolchain_library {
     name: "libatomic",
+    defaults: ["linux_bionic_supported"],
     arch: {
         arm: {
             instruction_set: "arm",
@@ -218,6 +219,7 @@ toolchain_library {
 
 toolchain_library {
     name: "libgcc",
+    defaults: ["linux_bionic_supported"],
     arch: {
         arm: {
             instruction_set: "arm",
@@ -227,6 +229,7 @@ toolchain_library {
 
 toolchain_library {
     name: "libgcov",
+    defaults: ["linux_bionic_supported"],
     arch: {
         arm: {
             instruction_set: "arm",