Fixes for host bionic support

This enables linux_bionic for llvm-headers-no-generated-headers, which
was missing.

Then add some more ifdefs in the host-side config.h (that's used for
linux_bionic) to deal with differences between bionic and glibc/darwin's
libc.

Bug: 31559095
Test: Enable host bionic, run soong
Change-Id: Ie01d3317eb21edd861163722d40a354207fada66
diff --git a/Android.bp b/Android.bp
index 0302e65..a29a38a 100644
--- a/Android.bp
+++ b/Android.bp
@@ -100,6 +100,9 @@
         host: {
             export_include_dirs: ["host/include"],
         },
+        linux_bionic: {
+            enabled: true,
+        },
         windows: {
             enabled: true,
         },