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/libm/Android.bp b/libm/Android.bp
index 6c3abd1..52229b6 100644
--- a/libm/Android.bp
+++ b/libm/Android.bp
@@ -509,7 +509,11 @@
     tidy_checks: ["-cert-dcl16-c"],
 
     include_dirs: ["bionic/libc"],
-    system_shared_libs: ["libc"],
+    target: {
+        bionic: {
+            system_shared_libs: ["libc"],
+        },
+    },
 
     sanitize: {
         address: false,