Investigate possible use of mask ROM for malloc/free

[cnlohr]

It has been a resounding success using the mask ROM for several libgcc functions. IF the mask ROM malloc functions can be safely used then many of these functions can be stubbed, others should be able to be directly linkable to the mask ROM. This should save additional space and help unify the system operation.

40100afc 0000001f T prvInsertBlockIntoUsedList
40100b34 0000007c T pvShowMalloc
40100b34 0000007c T system_show_malloc
40100bb0 00000029 T prvRemoveBlockFromUsedList
40100bdc 00000002 T xPortWantedSizeAlign
40100c08 0000015c T pvPortMalloc
40100d6c 0000008c T vPortFree
40100dfc 00000030 T pvPortCalloc
40100e2c 00000030 T pvPortCallocIram
40100e5c 00000019 T pvPortZalloc
40100e78 00000019 T pvPortZallocIram
40100e94 00000058 T pvPortRealloc
40100ef0 00000016 T malloc
40100f08 00000014 T free
40100f1c 00000014 T zalloc
40100f30 00000014 T calloc
40100f44 00000014 T realloc
40100f58 00000007 T xPortGetFreeHeapSize
40100f60 00000007 T xPortGetMinimumEverFreeHeapSize
40100fb0 000000f7 T vPortDefineHeapRegions

Document why we don't use, because I don't know!

40106a28 00000017 T memchr
40106a40 00000049 T __modsi3
40106a8c 00000018 T __ashrdi3