added mac arm branch

[GianFree]

On a mac M1, brew install coursier/formulas/coursier works installation-wise, while cs setup returns:

 ~ cs setup
The current machine does not support all of the following CPU features that are required by the image: [CX8, CMOV, FXSR, MMX, SSE, SSE2, SSE3, SSSE3, SSE4_1, SSE4_2, POPCNT, LZCNT, AVX, AVX2, BMI1, BMI2, FMA].
Please rebuild the executable with an appropriate setting of the -march option.

it seems the arm branch in the formula has been removed. Adding the branch and the url make it work.

[GianFree]

the sha256 of the arm branch should be checked.