Change arm64 platform to ARM64

[bpulliam]

Description

We have seen issues on the Arm64 platform arising from differences in capitalization, including long build times due to redundant build targets (arm64 vs ARM64). Because VC++ explicitly wants upper case (ARM64) whereas .NET doesn't care outside of runtime identifiers and Nuget architecture, upper case is what was chosen (the aforementioned exceptions are explicitly lower-cased in the handful of places where they are relevant).

Our recommendation for best practice is:

• Use Arm64 in documentation
• Use arm64 in RuntimeIdentifiers
• Use ARM64 everywhere else

Motivation and Context

The root cause is that our build was using inconsistent casing for the platform name (arm64 vs ARM64) which caused MSBuild to not recognize when multiple requested builds were building the same project with the same configuration and thus spawn multiple processes resulting in file access conflicts.

How Has This Been Tested?

Smoke testing

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)