libunw: Implement RISC-V stack unwinding

This patch implements stack unwinding for RISC-V architecture into
libunw. In RISC-V, the caller stores its frame pointer into register
s0 before calling the

libunw: Implement RISC-V stack unwinding

This patch implements stack unwinding for RISC-V architecture into
libunw. In RISC-V, the caller stores its frame pointer into register
s0 before calling the function. In the prologue of callee function,
the return address and the caller's frame pointer are saved into the
bottom of the callee's stack frame, and the callee's frame pointer is
also calculated and stored into s0 within the process of callee
function. Therefore, the caller's stack frame can be traced back from
the callee's stack frame.

The following steps describe the stack unwinding on RV64 system:
1. When an exception occurs, we get the current frame pointer from
trapped register s0/fp.
2. Get the ra from the memory address (fp-0x8).
3. Get the caller's frame pointer from the memory address (fp-0x10).
4. Update the caller's pc as (ra-0x4), this is information we want from
stack unwinding.
5. Repeat the step 2 to step 4, until we exceed the stack frame of the
thread.

Signed-off-by: Alvin Chang <alvinga@andestech.com>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@foss.st.com>

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