Use policies for code generation

Use policies for code generation

This patch changes code generation procedures add_code_header,
add_code_footer, and several other language-specific code generation
functions into policies.

gdb/ChangeLog:
        * compile/compile-c-support.c (add_code_header, add_code_footer):
        Move into policy class.
        (c_push_user_expression, pop_user_expression_nop)
        (c_add_code_header, c_add_code_footer, c_add_input): New policy class.
        (compile_program): New host class.
        (c_compile_program): New typedef.
        (c_compute_porgram): Use c_compile_program.