[toolchain] provide a cleaner way to specify a biarch toolchain build

[toolchain] provide a cleaner way to specify a biarch toolchain build

Currently, to build a biarch toolchain, we need to explicitly give
options to the binutils and gcc configure commands:

CONFIG_EXTRA_BINUTILS_CONFIG_OPTIONS="--enable-targets=powerpc64-linux-uclibc"
CONFIG_EXTRA_GCC_CONFIG_OPTIONS="--enable-biarch --enable-targets=powerpc64-linux-uclibc"

This change replaces the command line options with an 'extra arch'
configure option:

CONFIG_EXTRA_TARGET_ARCH=y
CONFIG_EXTRA_TARGET_ARCH_NAME="powerpc64"

And a way to invoke this extra arch on the compiler command-line:

CONFIG_EXTRA_TARGET_ARCH_OPTS="-m64"

In this case, this results in an extra compiler:
 'powerpc64-linux-uclibc-gcc', which invokes
 'powerpc-linux-uclibc-gcc -m64'

This is a more standard way of building biarch toolchains, and allows
the packages to not have to care about how to invoke the 64-bit
compiler.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@10802 3c298f89-4303-0410-b956-a3cf2f4a3e73