Initial support for SkylakeX / AVX512

This patch adds the basic infrastructure for adding the SkylakeX (Intel Skylake server)
target. The SkylakeX target will use the AVX512 (AVX512VL level) instruction set,
which brings 2 basic things:
1) 512 bit wide SIMD (2x width of AVX2)
2) 32 SIMD registers (2x the number on AVX2)

This initial patch only contains a trivial transofrmation of the Haswell SGEMM kernel
to AVX512VL; more will follow later but this patch aims to get the infrastructure
in place for this "later".

Full performance tuning has not been done yet; with more registers and wider SIMD
it's in theory possible to retune the kernels but even without that there's an
interesting enough performance increase (30-40% range) with just this change.

Add build-time configuration options to pkgconfig file

Change-Id: Ic896276cd073d6b41930c7c5a29d66348cd1725d

Without specifying thread count, NUM_THREADS would not be defined and CMake would fail.
This is because core count cannot be determined when cross-compiling.

These were inadvertently dropped from the list in my PR #1095

Allow overriding NUM_THREADS in cmake

MSVC or clang-cl.

* prebuild.cmake: Put quotes around path names that may contain whitespace
(Copied from alexkaratakis' PR #1295)
* kernel/CMakeLists.txt: Fix common_lapack header inclusion and DYNAMIC_ARCH generation of ?neg_tcopy and ?laswp_ncopy files
* lapack/CMakeLists.txt: Use correct template for ?laswp_(plus,minus) functions

Fix typos and bad if statements discovered in 0.2.20.

Add support for not having host compiler as CMake cannot detect such a compiler.
Add support for not using getarch.
Successfully builds Android ARMV8. Any target can be added by supplying the TARGET_CORE config in prebuild.cmake.

* Do not add -lpthread on Android builds

* Do not add -lpthread on Android cmake builds

Signed-off-by: Paul Osmialowski <pawel.osmialowski@arm.com>

* Multi-lib distributions need to change the libdir
  which is only portably possible with `GNUInstallDirs`.
* Multi-arch distributions such as Debian and Exherbo
  need to be able to change the bindir.

Update c_check.cmake to label ppc64 as power ARCH