[libc] Properly handle max malloc (=32764 bytes) in v7malloc

[ghaerr]

More v7malloc fixes paying proper attention to signed vs unsigned compares, 16-bit multiply overflow and address wrapping, all used within the allocation routine. This version properly handles the maximum allocation possible given it'ssbrk implementation and 16-bit int, which is malloc(32764).

Compiling with -Wextra also produced more warnings which are now fixed.

V7malloc is currently not used within any applications except fm, where it was required due to our current malloc fragmenting the heap. I am considering replacing the libc malloc with v7malloc. There is considerable difference in heap usage and fragmentation between malloc and realloc between the two implementations. Actual allocation speed performance is considered as less important than better heap management for our smaller address space.

[ghaerr]

Also corrected max allocation and errno handling in the standard ELKS libc malloc. This allows @ccoffing's malloc libc test to (finally) succeed.