sys-devel/crossdev forces multilib for arm toolchains

[kwhat]

Add support for more specific arm toolchain tripples allowing for USE=-multilib in some scenarios

Closes: https://bugs.gentoo.org/868636
Signed-off-by: Alexander Barker (https://github.com/kwhat) alex@1stleg.com

[kwhat]

This appears to be creating a working toolchain with minimal modification. I still want to do some more testing before considering this for merge, but review is welcome.

This requires a toolchain.eclass patch to work correctly due to the eabi[hs]f stuff.

crossdev-toolchain-eclass.patch

** Update 1:**

I have been driving this for a couple of day and it works but I had to build armv7m-none-eabihf with something like EXTRA_ECONF="--with-arch=armv7e-m --with-mode=thumb --with-fpu=fpv5-d16" to kind of get what I was expecting. It produces a compiler that does work without but some of the defaults are broken. I think the --with-fpu override is acceptable but I wonder if the toolchain eclass should make more of an effort to fill in --with-arch and --with-mode. I think there is enough here to address the bug filed; it is possible to make toolchains with these fixes here if the end user adds a little EXTRA_ECONF.