The backtrace code tries to look for an instruction of the form "sw ra, x(sp)"
to figure out the program counter of the calling function. When we generate
the kernel exception frame we store the 'ra' at the time of the exception
using an instruction of the same form. The problem is that the 'ra' at the
time of the exception is not the same as the 'program counter' at the time
of the exception.
The fix is to save the 'exception program counter' register by staging
it through the 'ra' register.
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