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/upstream-freebsd/lib/msun/src/s_fmin.c b/libm/upstream-freebsd/lib/msun/src/s_fmin.c
index d7f24c1..53f36c1 100644
--- a/libm/upstream-freebsd/lib/msun/src/s_fmin.c
+++ b/libm/upstream-freebsd/lib/msun/src/s_fmin.c
@@ -34,6 +34,13 @@
 
 #include "fpmath.h"
 
+#ifdef USE_BUILTIN_FMIN
+double
+fmin(double x, double y)
+{
+	return (__builtin_fmin(x, y));
+}
+#else
 double
 fmin(double x, double y)
 {
@@ -54,6 +61,7 @@
 
 	return (x < y ? x : y);
 }
+#endif
 
 #if (LDBL_MANT_DIG == 53)
 __weak_reference(fmin, fminl);