Added cross-compile targets for testing

Using gcc cross compilers and qemu:
- make test CC="arm-linux-gnueabi-gcc --static -mthumb" EXEC="qemu-arm"
- make test CC="powerpc-linux-gnu-gcc --static" EXEC="qemu-ppc"
- make test CC="mips-linux-gnu-gcc --static" EXEC="qemu-mips"

Also separated out Travis jobs and added some size reporting

Makefile

@@ -1,8 +1,8 @@
 TARGET = lfs
 
-CC = gcc
-AR = ar
-SIZE = size
+CC ?= gcc
+AR ?= ar
+SIZE ?= size
 
 SRC += $(wildcard *.c emubd/*.c)
 OBJ := $(SRC:.c=.o)