Mark CRT objects as vendor available

This will build separate copies of the CRT for vendor vs system when
BOARD_VNDK_VERSION is set. We'll be able to change crtbrand / etc as
needed with #ifdef __ANDROID_VNDK__ and similar.

Bug: 36426473
Bug: 36079834
Test: Put #ifdef __ANDROID_VNDK__ into crtbrand.S, see changes
Change-Id: I7db0cccfe94d144f072e7cef23832884f3c066f0
diff --git a/libc/Android.bp b/libc/Android.bp
index 688dd61..c7f52b1 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1849,6 +1849,7 @@
 cc_defaults {
     name: "crt_defaults",
     defaults: ["linux_bionic_supported"],
+    vendor_available: true,
 
     no_default_compiler_flags: true,
 
@@ -1878,6 +1879,7 @@
 cc_defaults {
     name: "crt_so_defaults",
 
+    vendor_available: true,
     arch: {
         mips: {
             cflags: ["-fPIC"],