Export bionic's resolv base64 functions to musl

Musl doesn't provide the resolv b64_* functions, but adb uses them.
Export them from bionic.

Bug: 190084016
Test: m USE_HOST_MUSL=true host-native
Change-Id: I37837e6179a15754d4cbd89e67649df9dea9d9f1
diff --git a/libc/Android.bp b/libc/Android.bp
index 63cdd35..83fe1db 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -2592,6 +2592,29 @@
     stl: "none",
 }
 
+cc_library_host_static {
+    name: "libb64",
+    visibility: ["//external/musl"],
+    srcs: ["upstream-openbsd/lib/libc/net/base64.c"],
+    export_include_dirs: ["b64/include"],
+    local_include_dirs: [
+        "private",
+        "upstream-openbsd/android/include",
+    ],
+    cflags: [
+        "-include openbsd-compat.h",
+    ],
+    enabled: false,
+    target: {
+        musl: {
+            enabled: true,
+            system_shared_libs: [],
+            header_libs: ["libc_musl_headers"],
+        },
+    },
+    stl: "none",
+}
+
 subdirs = [
     "bionic/scudo",
 ]