Permit allow_undefined_symbols inside arch blocks

Renderscript builds need to allow undefined symbols only for linux
host builds.

Test: mmma -j frameworks/compile/libbcc
Change-Id: Ieb83fc13fed16ae386261e0a1855fdd9c4fdf9bd
diff --git a/cc/linker.go b/cc/linker.go
index dac51d6..f4599e3 100644
--- a/cc/linker.go
+++ b/cc/linker.go
@@ -53,7 +53,7 @@
 	// modules cannot contain undefined symbols that are not satisified by their immediate
 	// dependencies.  Set this flag to true to remove --no-undefined from the linker flags.
 	// This flag should only be necessary for compiling low-level libraries like libc.
-	Allow_undefined_symbols *bool
+	Allow_undefined_symbols *bool `android:"arch_variant"`
 
 	// don't link in libgcc.a
 	No_libgcc *bool