While cross-compiling Linux kernel tools with Clang on ArchLinux, it
was revealed the sysroot is not properly detected and instead of fixing
the root cause in Clang, kernel developers started doing workarounds 
upon workarounds  in the kernel build to be able to pass a working
sysroot during Clang invocation.
To give a concrete example, ArchLinux installs its cross-compliation
sysroot in /usr/aarch64-linux-gnu for triplet aarch64-linux-gnu.
The sysroot is not properly detected in Linux::computeSysRoot() due
to the complicated code combining the GCC check with the mips test.
Fixing this bug allows to significantly clean up the kernel build as well
as to fix the cross-compilation detection on ArchLinux, as well as to
simplify the clang sysroot function.