Dotnet install-script updates

[johnbeisner]

Introduce support for arm & arm64 architectures; when detecting a missing 'library' prerequisite - do not fail the installation.

Addresses: https://github.com/dotnet/cli/issues/9007

Auto-detect tested successfully on:
OS = raspbian.8
Arch = armv7l

[natemcmaster]

Any reason we can't use uname -m to detect arch? Here are some sample values. I'm sure there are more. We could always err if we don't recognize the arch.

Platform	uname -m	Rid
macOS	x86_64	osx-x64
Ubuntu Bionic x64	x86_64	linux-x64
Ubuntu Bionic arm32v7	armv7l	linux-arm
Ubuntu Bionic arm64v8	aarch64	linux-arm64

[johnbeisner]

@natemcmaster
@janvorli
In the current version; the default architecture is 'x64' when "auto" is used.

I can see the value in trying to recognize the architecture; however, I feel strongly that we need to default to 'x64' when "auto" is used instead of an error-out. We would, at minimum, risk breaking fringe x64 Linux platforms if we now error-out on a peculiar return-value from 'uname -m'.

[johnbeisner]

@dotnet-bot Test RHEL7.2 x64 Release Build