get_fstar_z3: add command-line options to specify arch and kernel

[amosr-msft]

I need to make some Mac packages, but I don't have access to a corporation-blessed Mac machine. For cross-compiling or when we need to package up dependencies, it would be useful if the script to get Z3 could get specific architectures.
This PR adds --kernel and --arch options to get_fstar_z3.sh.

usage:

$ .scripts/get_fstar_z3.sh
Usage: get_fstar_z3.sh destination/directory/bin [--full] [--arch <x86_64|aarch64>] [--kernel <Linux|Darwin|Windows>]

example:

$ .scripts/get_fstar_z3.sh dest --arch x86_64 --kernel Windows
>>> Downloading Z3 4.8.5 from https://github.com/Z3Prover/z3/releases/download/Z3-4.8.5/z3-4.8.5-x64-win.zip ...
>>> Installed Z3 4.8.5 to dest/z3-4.8.5.exe
>>> Downloading Z3 4.13.3 from https://github.com/Z3Prover/z3/releases/download/z3-4.13.3/z3-4.13.3-x64-win.zip ...
>>> Installed Z3 4.13.3 to dest/z3-4.13.3.exe
>>> Downloading Z3 4.15.3 from https://github.com/Z3Prover/z3/releases/download/z3-4.15.3/z3-4.15.3-x64-win.zip ...
>>> Installed Z3 4.15.3 to dest/z3-4.15.3.exe

$ .scripts/get_fstar_z3.sh dest --arch x86_64 --kernel Linux
>>> Downloading Z3 4.8.5 from https://github.com/Z3Prover/z3/releases/download/Z3-4.8.5/z3-4.8.5-x64-ubuntu-16.04.zip ...
>>> Installed Z3 4.8.5 to dest/z3-4.8.5
>>> Downloading Z3 4.13.3 from https://github.com/Z3Prover/z3/releases/download/z3-4.13.3/z3-4.13.3-x64-glibc-2.35.zip ...
>>> Installed Z3 4.13.3 to dest/z3-4.13.3
>>> Downloading Z3 4.15.3 from https://github.com/Z3Prover/z3/releases/download/z3-4.15.3/z3-4.15.3-x64-glibc-2.39.zip ...
>>> Installed Z3 4.15.3 to dest/z3-4.15.3

[gebner]

The change to the script looks good to me.

But I'm not sure how this helps with mac support. Do you need x86_64 packages? x86_64 support on mac is essentially obsolete at this point.

FWIW, we have nightly and release builds for mac.