[arch-general] Change how cross compilers are packaged

[Christer Solskogen]

I've been using the aarch64 cross compiler packages for some time,
lately I've also taken a look at how that PKGBUILD looks and how it
relates to Cross-compiling tools package guidelines.

The situation now is that aarch64-linux-gnu-glibc installs into a
sysroot in /usr/aarch64-linux-gnu. Where also aarch64-linux-gnu-binutils
installs some files into. This is bad because the sysroot is a directory
thats supposed to be reserved for the target system.
This is fine on bare-metal systems, but not for linux/mingw targets.

What do you think of having /usr/sysroot/$target as sysroots instead for
   targets that are supposed to be configured that way? Only a small hand
full of packages needs to be rewritten (aarch64, mingw-w64 and riscv64)
- and I can fix that I (the package maintainers still need to review my
changes, as I don't have  git access)
One other option would be to follow gcc's defaults, which is 
$prefix/$target/sys-root

But before I do that, I'd like some comments if you think this is
needed. I don't want to waste time :-) At the same time, I would remove 
some of the old/deprecated options for binutils, glibc and gcc that are 
present in the current PKGBUILDs.

-- 
chs