make: use $(PREFIX)gcc instead of $(PREFIX)-gcc

The leading - makes it rather inconsistent with the majority of other
projects around the world.  Use the form everyone else uses.
To solve this, properly pass prefix to inner makes as was always
intended.

Fixes: https://github.com/libopencm3/libopencm3/issues/1058

lib/stm32/f7/Makefile

@@ -21,10 +21,8 @@
 LIBNAME		= libopencm3_stm32f7
 SRCLIBDIR	?= ../..
 
-PREFIX		?= arm-none-eabi
-
-CC		= $(PREFIX)-gcc
-AR		= $(PREFIX)-ar
+CC		= $(PREFIX)gcc
+AR		= $(PREFIX)ar
 
 # STM32F7 only supports single precision FPU
 FP_FLAGS	?= -mfloat-abi=hard -mfpu=fpv5-sp-d16