System.GC.AllocateUninitializedArray (#24096)

System.GC.AllocateUninitializedArray (#24096)

* Do not expand to allocation_quantum in SOH when GC_ALLOC_ZEROING_OPTIONAL

* short-circuit short arrays to use `new T[size]`

* Clean syncblock of large-aligned objects on ARM32

* specialize single-dimensional path  AllocateSzArray

* Unit tests

* Some PR feedback. Made AllocateUninitializedArray not be trimmed away.

* PR feedback on gchelpers
- replaced use of multiple bool parameters with flags enum
- merged some methods with nearly identical implementation
- switched callers to use AllocateSzArray vs. AllocateArrayEx where appropriate.

* PR feedback. Removed X86 specific array/string allocation helpers.