Remove the builtin import code

[antoyo]

The builtin import code is a hack and it's starting to be an issue.
With the more recent compiler_builtins which implements the math builtin functions in Rust, we can get conflicts between the GCC imported builtins and the Rust builtins:

libgccjit.so: error: : gcc_jit_function_new_block: cannot add block to an imported function
thread 'rustc' panicked at /home/user/.cargo/git/checkouts/gccjit.rs-13c2e290f2fb9e4d/f1545d7/src/function.rs:183:17:
gcc_jit_function_new_block: cannot add block to an imported function (cos)

[g4titanx]

if it's not being used anywhere else in the code, it shouldn't be a problem

[antoyo]

Well, it's used in the sense that it's how we are able to return the Function object for builtin functions when requested elsewhere.
But having to list them all is not a good solution: I'd prefer something that would fetch the builtin only when requested.
Feel free to ask for more info in that's not clear.