aout_BufferAlloc() : remove stack allocation

aout_BufferAlloc() : remove stack allocation

alloca() was not used anyway on OSX and BSD, due to smaller stack sizes,
and we can't assume a default stack size anyway

I expect the performance loss to be minimal, but worth the code
simplification anyway (i didn't benchmark)

aout_BufferAlloc() is moved into its own function in a .c file instead
of being a macro

Since there is now 2 types of allocations (HEAP and NONE), make
i_alloc_type a boolean (true = HEAP alloc, false = NO alloc)

make aout_BufferFree() a static inline function in the same process.
Prototype doesn't change since the provided buffer doesn't need to be
set to NULL (I checked all the callers)