Fix Canadian cross compilation

* install-sdk.sh: Remove the following flags: -a, --arch, --arm, --x86, -t
and --toolprfx. --host <arch-triplet> is sufficient now.
Use uname to figure out buildarch automatically.
Compare buildarch and hostarch to determine if we're cross-compiling. If we
are, make sure to set the right PATH so we use the buildarch->targetarch
compiler (and not hostarch->targetarch compiler).
Verify that buildarch->targetarch toolchain is present.
If we're doing Canadian cross, verify that buildarch->hostarch toolchain is
present.

* build-epiphany-sdk.sh: Pass hostarch triplet to install-sdk.sh when doing
Canadian cross compilation.

* build-toolchain.sh: Use uname to determine buildarch.
Install newlib for buildarch->targetarch compiler when doing Canadian cross
compilation. Some header files were missing (e.g. <sys/types.h>).