[WIP][RISC-V] Workaround to avoid crashes due to PMP protected memory.

[C4-S3]

As described in issue 105 the LiME module crashes under RISC-V when it
encounters a protected memory area through PMP (Physical Memory Protection).

In my workaround, I first test if the memory address is readable by
dereferencing it in a temporary-register. If an exception occurs during this test,
the error value is returned from the scause register. If the exception is a
"load access fault", zero bytes are written to the dump for the memory
page. This prevents the LiME module from crashing and as a small side effect
it is also possible to detect protected memory areas by PMP in S-Mode, since
the corresponding registers pmpaddr{0..n} can only be accessed in M-Mode.

[kd8bny]

Setting up a risc v env