minimal: Use soft float for CROSS=1 Cortex-M4 target.

When compiled with hard float the system should enable FP access when it
starts or else FP instructions lead to a fault.  But this minimal port does
not enable (or use) FP and so, to keep it minimal, switch to use soft
floating point.  (This became an issue due to the recent commit
34c04d2319 which saves/restores FP registers
in the NLR state.)

ports/minimal/Makefile

@@ -19,7 +19,7 @@ INC += -I$(BUILD)
 ifeq ($(CROSS), 1)
 DFU = $(TOP)/tools/dfu.py
 PYDFU = $(TOP)/tools/pydfu.py
-CFLAGS_CORTEX_M4 = -mthumb -mtune=cortex-m4 -mabi=aapcs-linux -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=hard -fsingle-precision-constant -Wdouble-promotion
+CFLAGS_CORTEX_M4 = -mthumb -mtune=cortex-m4 -mcpu=cortex-m4 -msoft-float -fsingle-precision-constant -Wdouble-promotion
 CFLAGS = $(INC) -Wall -Werror -std=c99 -nostdlib $(CFLAGS_CORTEX_M4) $(COPT)
 LDFLAGS = -nostdlib -T stm32f405.ld -Map=$@.map --cref --gc-sections
 else