Fixes for PGI compiler

fixes #2149

Add gfortran workaround for C->FORTRAN ABI violation

for #2154 (see gcc bug 90329)

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.

for calling from concurrent threads

for #2033

fixes #2033

fixes #1961

the build will now abort with an error message when an undefined build TARGET is named

Fixes #1938

override user-provided FFLAGS if necessary

FreeBSD uses powerpc64 name for POWER architecture. Use correct Makefile for this platform.

Set LIBSONAME suffix to .a for AIX

another fix for #1803

Enable DYNAMIC_ARCH feature on ARM64. This patch uses the cpuid
feature in linux kernel to detect the core type at runtime
(https://www.kernel.org/doc/Documentation/arm64/cpu-feature-registers.txt).

If this feature is missing in kernel, then the user should use the
OPENBLAS_CORETYPE env variable to select the desired core type.

Handle conflicting usage of ARCH in at least some BSD environments

fixes #1796

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.

Fix race condition in blas_server_omp.c

Change -openmp to -fopenmp for ifort entry as well

Update compiler flag for openmp use with ICC

The deprecated -openmp option was finally removed in favor of -qopenmp or -fopenmp, picking the latter to stay compatible with Intel compiler versions before 2015 (when -q options were introduced). Fixes #1546

Change-Id: Ic896276cd073d6b41930c7c5a29d66348cd1725d

With this change, OpenBLAS builds and all tests pass on OpenBSD 6.2
using Clang. Tested on x86-64 only, with and without DYNAMIC_ARCH=1.