riscv64: Fix underflow in call relocation handling

Under some test case layouts the call relocation
panicking with an underflow. Use `wrapping_sub` to
signal that this is expected.

The fuzzer took a while to generate such a test case.
And I can't introduce it as a regression test because
when running via the regular clif-util run tests the
layout is different and the test case passes!

I think this is because in the fuzzer we only add
one trampoline, while in clif-util we build trampolines
for each funcion in the file.

cranelift/jit/src/compiled_blob.rs

@@ -123,7 +123,7 @@ impl CompiledBlob {
                     // Unlike the regular symbol relocations, here both "sub-relocations" point
                     // to the same address.
                     let hi20 = pcrel.wrapping_add(0x800) & 0xFFFFF000;
-                    let lo12 = (pcrel - hi20) & 0xFFF;
+                    let lo12 = pcrel.wrapping_sub(hi20) & 0xFFF;
 
                     unsafe {
                         // Do a R_RISCV_PCREL_HI20 on the `auipc`