buildtools: move to working directory

There is absolutely *no* reason for the buildtools (wrappers to gcc, g++,
as, ld... for the local machine) to be in the toolchain directory. Moreover,
they are removed after the build completes.

Move them out of the toolchain directory, and into the build directory (but
yet the part specific to the current toolchain). This means we no longer
need to explicitly remove them either, BTW, but we need to save/restore them
for the restart feature.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # x86 specific options

 config ARCH_x86

     select ARCH_SUPPORTS_32

     select ARCH_SUPPORTS_64

     select ARCH_DEFAULT_32

     select ARCH_USE_MMU

     select ARCH_SUPPORT_ARCH

     select ARCH_SUPPORT_CPU

     select ARCH_SUPPORT_TUNE

     help

       The x86 architecture, as defined by:

         32-bit (ia32) : http://www.intel.com/

         64-bit (amd64): http://www.amd.com/us-en/Processors/DevelopWithAMD/0,,30_2252_875_7044,00.html