Replace Neo.Windows with clauses with Neo.Linux
Linux: Implement Get_Number_Of_Cores

Use the standard System.Multiprocessoros package to determine the number of
available CPUs.
Add CPUID availability check for x86-64

The else block basically duplicates the code for the case when USE_64_BIT is set
to False. The only adjustment is to use appropriately sized registers.

Comment: It is unclear to me, why source in the x86-64-specific directory would
need to check the USE_64_BIT flag since x86-64 is inherently 64-bit. The check
and the unneeded code can probably be dropped.
Move Record_Memory_Status type to Neo.System.Memory

Since this type is also needed in the Linux-specific
Neo.System.Memory.Implementation package, move the type to a common package.

This allows the build to succeed on Linux.
Add 'main' to .gitignore file
Update neo-system-processor-implementation_for_operating_system.adb

Multiprocessors.Number_Of_CPUs is the fallback called by the processor main body already. Throwing an error already makes it get called.
(https://github.com/AdaDoom3/AdaDoom3/blob/linux/Code/neo-system-processor.adb)
Merge pull request #6 from AdaDoom3/linux

Linux