auto merge of #6249 : crabtw/rust/arm, r=brson

It uses the private field of TCB head to store stack limit. I tested on my Raspberry PI. A simple hello world program ran without any problem. However, for a more complex program, it segfaulted as #6231.

diff --cc mk/platform.mk
Simple merge

diff --cc mk/rt.mk
Simple merge

diff --cc src/rt/rust_task.h
index 672af608db86358053ad1880a5897f3371547999,6813e3b46c458e0f6a38760714293d404ba581e1..213aee6a9ebccac70ef671b028a8486a20302705
--- 1/src/rt/rust_task.h
--- 2/src/rt/rust_task.h
+++ b/src/rt/rust_task.h
@@@ -144,8 -144,11 +144,11 @@@
  #define RED_ZONE_SIZE RZ_LINUX_64
  #endif
  #ifdef __mips__
 -#define RED_ZONE_SIZE RZ_LINUX_32
 +#define RED_ZONE_SIZE RZ_MAC_32
  #endif
+ #ifdef __arm__
+ #define RED_ZONE_SIZE RZ_LINUX_32
+ #endif
  #endif
  #ifdef __APPLE__
  #ifdef __i386__