Add support for a new function attribute

To add support for a new function attribute in libgccjit, you need to do the following steps:

1. Copy the corresponding function from c-family/c-attribs.cc into jit/dummy-frontend.cc. For example if you add the target attribute, the function name will be handle_target_attribute.
2. Copy the corresponding entry from the c_common_attribute_table variable in the c-family/c-attribs.cc file into the jit_attribute_table variable in jit/dummy-frontend.cc.
3. Add a new variant in the gcc_jit_fn_attribute enum in the jit/libgccjit.h file.
4. Add a test to ensure the attribute is correctly applied in gcc/testsuite/jit.dg/. Take a look at gcc/testsuite/jit.dg/test-nonnull.c if you want an example.
5. Run the example like this (in your gcc-build folder): make check-jit RUNTESTFLAGS="-v -v -v jit.exp=jit.dg/test-nonnull.c"

Once done, you need to update the gccjit.rs crate to add the new enum variant in the corresponding enum (FnAttribute).

Finally, you need to update this repository by calling the relevant API you added in gccjit.rs.

To test it, build gcc, run cargo update -p gccjit and then you can test the generated output for a given Rust crate.