Refactor makefile

[chenyukang]

I found boot/x86_64/entry64.o and boot/x86_64/main.o was checkin,
removing them got making error,
then I found boot/x86_64 is not called for making.
Do some refactors on Makefiles,

Bad effect for this patching is we need two more exporting for Mac,
It's should have no effects on Linux.

add this exporting demo to wiki should be OK:

export RUST_ROOT=/Users/yukang/code/nightly
export LINKEDITOR=/usr/local/gcc-4.8.1-for-linux64/bin/x86_64-pc-linux-ld
export ARCHIVE=/usr/local/gcc-4.8.1-for-linux64/bin/x86_64-pc-linux-ar

export CC=/usr/local/gcc-4.8.1-for-linux64/bin/x86_64-pc-linux-gcc
export OBJCPY=/usr/local/gcc-4.8.1-for-linux64/bin/x86_64-pc-linux-objcopy

[ckkashyap]

hmm ... in addition to the environment variable, it also requires someone on mac to have gcc cross compiler.
Eventually, we should replace the whole thing under boot with some implementation in assembly. We can relook at this change at that point.

[chenyukang]

OK.