Revert "Fix malloc corrupting the stack. Added new function reserve_stack() to adjust the amount of memory reserved for the stack."

Different version applied upstream.

This reverts commit c5bdda910b.

teensy3/kinetis.h

 extern void (* _VectorsRam[NVIC_NUM_INTERRUPTS+16])(void);
 extern void (* const _VectorsFlash[NVIC_NUM_INTERRUPTS+16])(void);
 
-// Reserve stack memory. This is the amount of memory malloc will leave alone
-// for the stack. The stack can grow larger than this limit (growing towards /
-// into the heap). 512 bytes are reserved by default.
-extern void reserve_stack(int size);
-
 #ifdef __cplusplus
 }
 #endif

teensy3/mk20dx128.c

 #include "core_pins.h" // testing only
 #include "ser_print.h" // testing only
 
-#include <errno.h>
 
 // Flash Security Setting. On Teensy 3.2, you can lock the MK20 chip to prevent
 // anyone from reading your code.  You CAN still reprogram your Teensy while
 
 char *__brkval = (char *)&_ebss;
 
-int reserved_stack_size = 512;
-
-void reserve_stack(int size)
-{
-    reserved_stack_size = size;
-}
-
 void * _sbrk(int incr)
 {
-    if(__brkval + incr >= (char*) &_estack - reserved_stack_size)
-    {
-        errno = ENOMEM;
-        return (void*) -1;
-    }
 	char *prev = __brkval;
 	__brkval += incr;
 	return prev;