Move system_shared_libs into target.bionic clause

Use target.bionic.system_shared_libs when it is used to limit the
default shared libraries (as opposed to remove them completely).
This avoids attempting to add a host dependency on libc when
system_shared_libs is modified to apply to all variants.

Bug: 193559105
Test: m checkbuild
Change-Id: I8d623321ce5145cf2968f83f78b5cb50711290f4
diff --git a/libc/Android.bp b/libc/Android.bp
index d24e387..4dfabaf 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1983,8 +1983,12 @@
     static: {
         system_shared_libs: [],
     },
-    shared: {
-        system_shared_libs: ["libc"],
+    target: {
+        bionic: {
+            shared: {
+                system_shared_libs: ["libc"],
+            },
+        },
     },
 
     //TODO (dimitry): This is to work around b/24465209. Remove after root cause is fixed