lib/libm_dbl: Add round.c source code.

This code is imported from musl, to match existing code in libm_dbl.

The file is also added to the build in stm32/Makefile.  It's not needed by
the core code but, similar to c5cc6417,
allows round() to be used by user C modules or board extensions.

lib/libm_dbl/round.c

+#include "libm.h"
+
+#if FLT_EVAL_METHOD==0 || FLT_EVAL_METHOD==1
+#define EPS DBL_EPSILON
+#elif FLT_EVAL_METHOD==2
+#define EPS LDBL_EPSILON
+#endif
+static const double_t toint = 1/EPS;
+
+double round(double x)
+{
+	union {double f; uint64_t i;} u = {x};
+	int e = u.i >> 52 & 0x7ff;
+	double_t y;
+
+	if (e >= 0x3ff+52)
+		return x;
+	if (u.i >> 63)
+		x = -x;
+	if (e < 0x3ff-1) {
+		/* raise inexact if x!=0 */
+		FORCE_EVAL(x + toint);
+		return 0*u.f;
+	}
+	y = x + toint - toint - x;
+	if (y > 0.5)
+		y = y + x - 1;
+	else if (y <= -0.5)
+		y = y + x + 1;
+	else
+		y = y + x;
+	if (u.i >> 63)
+		y = -y;
+	return y;
+}

ports/stm32/Makefile

@@ -191,6 +191,7 @@ LIBM_SRC_C = $(addprefix lib/libm_dbl/,\
 	nearbyint.c \
 	pow.c \
 	rint.c \
+	round.c \
 	scalbn.c \
 	sin.c \
 	sinh.c \