Audit usages of Marshal.AllocHGlobal for ones that can be replaced with NativeMemory.Alloc

#54006 introduced new NativeMemory APIs that wrap malloc, calloc, realloc, and aligned_alloc. We should audit existing usages of Marshal.AllocHGlobal for cases where it can be safely replaced with malloc.

Notably cases like StringToPtrHGlobal do not qualify as they are documented to be a call to LocalAlloc on Windows. However, on Unix and in cases where this is not a definitive contract, it should be safe to replace these calls with faster ones to NativeMemory.