stm32l4: enable FPU in chipset specific startup.

Helpful if you don't like seeing:
(gdb) vecstate
HardFault: forced due to escalated or disabled configurable fault (see below)
UsageFault due to access to disabled/absent coprocessor

lib/stm32/l4/vector_chipset.c

@@ -0,0 +1,27 @@
+/*
+ * This file is part of the libopencm3 project.
+ *
+ * Copyright (C) 2010 Piotr Esden-Tempski <piotr@esden.net>
+ * Copyright (C) 2011 Fergus Noble <fergusnoble@gmail.com>
+ *
+ * This library is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this library.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <libopencm3/cm3/scb.h>
+
+static void pre_main(void)
+{
+	/* Enable access to Floating-Point coprocessor. */
+	SCB_CPACR |= SCB_CPACR_FULL * (SCB_CPACR_CP10 | SCB_CPACR_CP11);
+}