(and other small fixes to match Makefile behaviour)

21eda8b5 introduced a check in getarch.c to test if the compiler is capable of
AVX512. This check currently fails, since the used __AVX2__ macro is only
defined if getarch itself was compiled with AVX2/AVX512 support. Make sure this
is the case by building getarch with -march=native on x86_64. It is only
supposed to run on the build host anyway.

Found via `codespell -q 3 -w -L ith,als,dum,nd,amin,nto,wis,ba -S ./relapack,./kernel,./lapack-netlib`

e.g. cmake -DDYNAMIC_ARCH=1 -DDYNAMIC_LIST="NEHALEM;HASWELL;ZEN" ..
original issue was #1639

Should fix 1806 and #1801

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.

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.

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.

If OpenBLAS is built using add_subdirectory(OpenBlas) as part of another project
then the paths set by CMAKE_XXX_DIR are relative to the parent project
and not the OpenBLAS project.

e.g. Build 32-bit on 64-bit Linux.
cmake -DBINARY=32

Disable CBLAS and LAPACK.

This will generate obj files for SINGLE/DOUBLE/COMPLEX/DOUBLE COMPLEX.

Fixed some typos.

Updated c_check to set ARCH and BINARY64/32.

Added version variables.

Still isn't parsed into CMake variables, and getarch_2 needs to
get the same treatment.

It only does the double precision compile currently.

I realized I didn't finish converting Makefile.system yet, so I made
a note of that.