Add mpn_sqr and mpn_mulhigh routines for Arm

[albinahlback]

And cleanup some code for related tests.

Added hardcoded squaring routines for $n < 10$ and added high multiplication routines, hardcoded ones for $n \le 8$ and then a basecase one.

Hopefully this Arm routine also works for Apple.

Also fixed mulhigh tests to check against a generic version, so that all architecture behaves the same.

[albinahlback]

I hope I didn't screw up preprocessor stuff in mpn_extras.h.

[albinahlback]

Btw, @fredrik-johansson, I only want us to define FLINT_HAVE_NATIVE_xxx if there is a native function for xxx (i.e. it is written in assembly).

[albinahlback]

Btw, here is some timings from an M1 running Debian profiling mulhigh:

        mul_n / mulhigh_n || mpfr / flint
n =  1:     0.885x        ||    5.292
n =  2:     1.256x        ||    5.759
n =  3:     1.451x        ||    4.383
n =  4:     1.512x        ||    3.481
n =  5:     1.556x        ||    3.351
n =  6:     1.594x        ||    3.250
n =  7:     1.639x        ||    3.019
n =  8:     1.558x        ||    2.254
n =  9:     1.437x        ||    1.758
n = 10:     1.437x        ||    1.622
n = 11:     1.390x        ||    1.545
n = 12:     1.406x        ||    1.531
n = 13:     1.376x        ||    1.512
n = 14:     1.355x        ||    1.428
n = 15:     1.333x        ||    1.386
n = 16:     1.397x        ||    1.368
n = 17:     1.372x        ||    1.783
n = 18:     1.363x        ||    1.859
n = 19:     1.388x        ||    1.784
n = 20:     1.310x        ||    1.590
n = 21:     1.306x        ||    1.565
n = 22:     1.248x        ||    1.530
n = 23:     1.294x        ||    1.333
n = 24:     1.274x        ||    1.667
n = 25:     1.293x        ||    1.639
n = 26:     1.241x        ||    1.352
n = 27:     1.262x        ||    1.349
n = 28:     1.264x        ||    1.599
n = 29:     1.256x        ||    1.342
n = 30:     1.237x        ||    1.327

[albinahlback]

Hardcoded sqrhigh routines:

         sqr / sqrhigh || mpfr / flint
n = 1:     1.006x      ||    5.956
n = 2:     1.061x      ||    3.139
n = 3:     1.243x      ||    2.463
n = 4:     1.248x      ||    2.381
n = 5:     1.409x      ||    2.591
n = 6:     1.524x      ||    2.910
n = 7:     1.342x      ||    2.728
n = 8:     1.752x      ||    3.268