Apart from bumping the toolchain Clang's and port's version, this commit
completely overhauls the way LLVM toolchain is built.
First, it gets rid of a complicated two-stage process of first compiling
clang and compiler-rt builtins and then building libunwind, libc++abi,
and libc++ -- it is possible to create a complete cross-compilation
toolchain in a single CMake invocation with a modern LLVM. Moreover, the
old method was inherently unsupported and subtly broken.
Next, it utilizes full potential of the Stubs "framework". Now we are
even able to compile Clang with -Wl,-z,defs which makes one of the
patches obsolete and the whole installation less error-prone. Note that
it comes at a cost of complicating the bootstrap process on a completely
novel architecture but this hopefully won't happen often.
Lastly, it fixes handling of the -no*lib* family of flags in the
Serenity LLVM driver and correctly uses -nostartfiles in conjunction
with stubs to make necessary CMake configure-time checks succeed.
This allows the linker to link against these dynamic libraries when
compiling libc++/libunwind, without having to do a separate
bootstrapping LibC build.
Without this change, libc++ would fail to pick up the need to link to
`LibPthread` if no prior builds of it existed. Because of this, we'd
immediately have an assertion failure in SystemServer, as mutexes are
used for the safe construction of function-local static variables.
