Add a exception handling helper code

This commit add a structure definition that represents the stack frame
layout as it is upon entering exception handler. This is useful when
one wants to alter the exception return address, or get information
about registers state saved by the CPU.
Add a "noreturn" attribute to 'scb_reset_system'

Adding this attribute allows to avoid warnings issued by GCC in cases
when 'scb_reset_system' is used as a last call in a function with
"noreturn" attribute set(usually reset handler of some sorts)
Fix compile time warnings for 'scb_reset_system'

Adding 'noreturn' to the function cases GCC to issue a warning, since
it still adds standard function's prologue/epilogue. Adding 'naked'
attribute fixes that.
Merge pull request #29 from aspalmer/scb

Scb