arm: refactor fault handling stack pointer passing

Find out on which stack the stack frame for an exception is in the assembly
code (__fault()) rather than in C (_Fault()). This will allow pushing
more registers on the stack when debugging is enabled.

Change-Id: I1c510b83098536f8930392b17df27511ccd04d80
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>