riscv: Remove RISCV_GCC_RED_ZONE_SIZE

The current ABI says that there is no stack red zone:

https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md

"Procedures must not rely upon the persistence of stack-allocated data
whose addresses lie below the stack pointer."

Update #3433.

cpukit/score/cpu/riscv/include/rtems/score/cpu.h

@@ -164,9 +164,6 @@ uint32_t _CPU_ISR_Get_level( void );
 
 /* end of ISR handler macros */
 
-/* Context handler macros */
-#define RISCV_GCC_RED_ZONE_SIZE 128
-
 void _CPU_Context_Initialize(
   Context_Control *context,
   void *stack_area_begin,

cpukit/score/cpu/riscv/riscv-context-initialize.c

@@ -50,8 +50,7 @@ void _CPU_Context_Initialize(
 {
   uintptr_t stack = ((uintptr_t) stack_area_begin);
 
-  /* Account for red-zone */
-  uintptr_t stack_high = stack + stack_area_size - RISCV_GCC_RED_ZONE_SIZE;
+  uintptr_t stack_high = stack + stack_area_size;
 
   memset(context, 0, sizeof(*context));