External toolchain support

Hi all,

It is now possible to use an external toolchain to build the kernel and base
(tested with gcc 4.9.1 and latest binutils)

Of course a lot of work is needed to make it build cleanly (aka lots of warning
to fix).

What have been tested so far:
- sparc64 kernel + world
- amd64 kernel + world
- powerpc64 kernel + world

mips cannot be tested because upstream gcc never heard of FreeBSD running on
mips, and I did not receive any patches for mips.

for amd64, in the kernel two things had to be removed from the build:
- aesni: (it request a header which is compiler specific and on recent gcc
will end up including stdlib.h which gives errors because kernel version of free
and malloc are not compatible with the version defined in stdlib.h)
- hptmv: I had to remove it from GENERIC and kernel building.

The result is:

$ sysctl kern.ostype kern.osrelease kern.osrevision kern.compiler_version

kern.ostype: FreeBSD
kern.osrelease: 11.0-CURRENT
kern.osrevision: 199506
kern.compiler_version: gcc version 4.9.1 (FreeBSD Ports Collection for amd64)

so yes it boots and runs

How to do you own testing:
in the ports tree/packages (the amd64 version will appear in packages next week)
install:
amd64-xtoolchain-gcc or powerpc64-xtoolchain-gcc or sparc64-xtoolchain-gcc

if your source tree:
make CROSS_TOOLCHAIN=amd64-gcc -j8 buildkernel
or
make CROSS_TOOLCHAIN=powerpc64-gcc -j8 buildkernel
or
make CROSS_TOOLCHAIN=sparc64-gcc -j8 buildkernel

To build world:
same operation with buildworld. Please note that for world you will need to add
define NO_WERROR (world will also require a change in share/mk/bsd.lib.mk:
s/--fatal-warnings/--no-fatal-warnings/)

also notes that for the kernel a lots of warnings are disabled in
share/sys/kern.mk so do not hesitate to remove yourself those -Wno-error= and
fix the issue they are hidding!

Best regards,
Bapt