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/vf6xx/Makefile

@@ -22,10 +22,8 @@ LIBNAME		= libopencm3_vf6xx
 SRCLIBDIR	?= ..
 
 FP_FLAGS	?= -mfloat-abi=hard -mfpu=fpv4-sp-d16
-PREFIX		?= arm-none-eabi
-
-CC		= $(PREFIX)-gcc
-AR		= $(PREFIX)-ar
+CC		= $(PREFIX)gcc
+AR		= $(PREFIX)ar
 TGT_CFLAGS	= -Os \
 		  -Wall -Wextra -Wimplicit-function-declaration \
 		  -Wredundant-decls -Wmissing-prototypes -Wstrict-prototypes \