arm64: use llvm's builtins for copysign() and copysignf().

Test: objdump
Change-Id: I26ab2c4745c5f64140cf781a9f42bce6fc79e1ef
diff --git a/libm/Android.bp b/libm/Android.bp
index 6249aa1..98c89dc 100644
--- a/libm/Android.bp
+++ b/libm/Android.bp
@@ -322,6 +322,8 @@
                 "upstream-freebsd/lib/msun/src/e_sqrtf.c",
                 "upstream-freebsd/lib/msun/src/s_ceil.c",
                 "upstream-freebsd/lib/msun/src/s_ceilf.c",
+                "upstream-freebsd/lib/msun/src/s_copysign.c",
+                "upstream-freebsd/lib/msun/src/s_copysignf.c",
                 "upstream-freebsd/lib/msun/src/s_floor.c",
                 "upstream-freebsd/lib/msun/src/s_floorf.c",
                 "upstream-freebsd/lib/msun/src/s_fma.c",