Suport passing arbitrary llvm arguments (#271)

- Support for passing LLVM command line options via the prcoess input or
providing one or more `--llvm-arg='..'` resolc CLI flag. This allows
more fine-grained control over the LLVM backend configuration.
- Make LLVM initialization idempotent.

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>

crates/solidity/src/process/mod.rs

@@ -37,11 +37,19 @@ pub trait Process {
         }
 
         let input: Input = revive_common::deserialize_from_slice(buffer.as_slice())?;
+
+        revive_llvm_context::initialize_llvm(
+            revive_llvm_context::Target::PVM,
+            crate::DEFAULT_EXECUTABLE_NAME,
+            &input.llvm_arguments,
+        );
+
         let result = input.contract.compile(
             input.project,
             input.optimizer_settings,
             input.include_metadata_hash,
             input.debug_config,
+            &input.llvm_arguments,
         );
 
         match result {