Reduce scope of AVX-512 C flags

[brada4]

Should fix #1909
Not sure if it spots cygwin correctly still
Probably -march=skylake-avx512 can be replaced with -mavx512vl

[martin-frbg]

see #1910. Your idea of putting it into the FORCE_SKYLAKEX section is more elegant, but I expect it will indeed cause problems on Cygwin (mine is not perfect in this regard either, I guess I should copy the extra handling to the kernel Makefile)

[brada4]

Each kernel type sort of deserves own -march flag (plus workarounds)
I dont see elegant way to put all in toplevel. Maybe -march=native if !DYNAMIC_ARCH !FORCE_TARGET ? What do you think?
EDIT: #1910 correctly keeps native SKYLAKEX build native with no impact on others.
I dont know what kicks old architectures will get from $FC -march=native , back then with gcc it was 10-20%, but not for numeric code.

[brada4]

Testing alpine myself now.
Feel free to commit yours and I will adjust mine to fit over it.

[brada4]

I will not be taking care of this until January, problem lies in build system compiling universal subarch codes with detected target flags, Same applies to arm(64), so there is a need for extra set of absolutely generic cflags for common code.