Take upstream libm changes.

Mostly workarounds for GCC and Clang bugs.

Change-Id: I4ef428a42d4ac6d622659053711a8cc416925727
diff --git a/libm/upstream-freebsd/lib/msun/src/e_log2f.c b/libm/upstream-freebsd/lib/msun/src/e_log2f.c
index 7166346..1794484 100644
--- a/libm/upstream-freebsd/lib/msun/src/e_log2f.c
+++ b/libm/upstream-freebsd/lib/msun/src/e_log2f.c
@@ -26,6 +26,7 @@
 ivln2lo    = -1.7605285393e-04; /* 0xb9389ad4 */
 
 static const float zero   =  0.0;
+static volatile float vzero = 0.0;
 
 float
 __ieee754_log2f(float x)
@@ -38,7 +39,7 @@
 	k=0;
 	if (hx < 0x00800000) {			/* x < 2**-126  */
 	    if ((hx&0x7fffffff)==0)
-		return -two25/zero;		/* log(+-0)=-inf */
+		return -two25/vzero;		/* log(+-0)=-inf */
 	    if (hx<0) return (x-x)/zero;	/* log(-#) = NaN */
 	    k -= 25; x *= two25; /* subnormal number, scale up x */
 	    GET_FLOAT_WORD(hx,x);