Make bionic_platform_headers available only for Bionic targets.

The triggering reason for this change is that the SDK snapshot
generation cannot readily handle header libraries that are required for
targets that the SDK itself isn't available for. However, these include
files shouldn't be used without Bionic anyway.

Test: m checkbuild (on aosp_taimen)
Test: m (on aosp_taimen_hwasan)
Test: atest CtsBionicTestCases
Test: lunch fvp-userdebug && mmm bionic
Bug: 152255951
Change-Id: Icdc49558893c5355860f78d23275c49ba0119900
diff --git a/libc/Android.bp b/libc/Android.bp
index f73ae4a..b2ba2c3 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1751,6 +1751,7 @@
 // Headers that only other parts of the platform can include.
 cc_library_headers {
     name: "bionic_libc_platform_headers",
+    defaults: ["linux_bionic_supported"],
     visibility: [
         "//art:__subpackages__",
         "//bionic:__subpackages__",
@@ -1762,7 +1763,6 @@
         "//system/core/libunwindstack:__subpackages__",
         "//system/memory/libmemunreachable:__subpackages__",
     ],
-    host_supported: true,
     vendor_available: true,
     ramdisk_available: true,
     recovery_available: true,