libm: sync with upstream.

There's potential here to maybe lose some/all of builtins.cpp, but I'll
look at that separately later.

Test: treehugger
Change-Id: I2c2bc1d0753affdd214daeb09fa1ac7cd73db347
diff --git a/libm/freebsd-compat.h b/libm/freebsd-compat.h
index f400f2a..7accc55 100644
--- a/libm/freebsd-compat.h
+++ b/libm/freebsd-compat.h
@@ -39,3 +39,7 @@
 
 // Similarly rename _scan_nan.
 #define _scan_nan __libm_scan_nan
+
+// FreeBSD exports these in <math.h> but we don't.
+double cospi(double);
+double sinpi(double);