Move TLS key deletion to openblas_quit

fixes #1954 (as suggested by thrasibule in that issue)

Query AVX2 and AVX512VL support when selecting x86 kernels

as homebrew seems to have dropped support for El Capitan in their gcc packages

More fixes for cross-compiling ARM64 targets

Fixed core naming for DYNAMIC_ARCH. Corrected GEMM_DEFAULT entries and added SYMV_P. Replaced outdated VULCAN define for ThunderX2T99 with ARMV8 to get basic definitions back. For issue #1908

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

Fixes #1938

Typo: Skyalke -> Skylake

Worth fixing, it gets in the way of searching....

Fix typo in UNKNOWN core name

Re-enable loop unrolling in trmv and remove the scary warning

fixes #1748 as that half of the fix for #1332 appears to have been an overreaction on my part.

Delete the pthread key on cleanup in TLS mode

already provided conditionally by common.h via openblas_utest.h
Unconditional inclusion breaks older Android and similar platforms that use OPENBLAS_COMPLEX_STRUCT

to avoid a crash when OpenBLAS was loaded via dlopen and libc tries to clean up the leaked TLS after dlclose
Fixes #1720

Should be of no consequence, right?

Add -fPIC to provided CFLAGS/FFLAGS if required

Fix thinko in skylake beta handling

casting ints is cheaper but it has a rounding, not memory casing effect, resulting in
invalid outcome

override user-provided FFLAGS if necessary

Add -mavx2 to TARGET=HASWELL builds

to leverage improvements from PR#1921

Reflect ARMV8 target definition changes from PR1876

and create config target directory for cross-compiles.

Avoid taking the root of a negative number in simple threaded syrk

Fixes #1924 where numpy 1.17+ would report the (transient) FE_INVALID exception raised for the domain error.

rebase