Fix overallocation of arm64 small constant localloc

Fix overallocation of arm64 small constant localloc

We were dividing the number of bytes to allocate by 8 (right shift by 3)
to determine the number of loops, then allocating and zeroing 16 bytes
per loop.

Simplify this by removing the number of `STACK_ALIGN_SHIFT` cases we have.

Add a small test case that exhibits this behavior.

Fixes dotnet/coreclr#4571

Commit migrated from https://github.com/dotnet/coreclr/commit/7cd8f70d30963df4aa85203ba5f39f41285b2cd3