Fix extraction of shift value for immediate instructions

The shift value must be treated as an unsigned integer otherwise
a shift value such as '0b11111' would be interpreted as -1 instead
of 31.

angr_platforms/risc_v/instrs_riscv/instruction_patterns.py

@@ -94,8 +94,8 @@ def get_imm(self):
         return self.constant(data, Type.int_32)
 
     def get_shift_amount(self):
-        num = BitArray(bin=self.data['I']).int
-        return self.constant(num, Type.int_8)
+        num = BitArray(bin=self.data['I'])
+        return self.constant(num.uint, Type.int_8)
 
     def get_optional_func7(self):
         return self.data['i']