Code:
extern uint8_t code_gen_prologue[];
This is declaration of a variable with external linkage named "code_gen_prologue" which is an array of uint8_t (single-byte unsigned integer). It just informs the compiler that there is such a variable.
Code:
uint8_t code_gen_prologue[1024] code_gen_section;
This is definition of the above variable (here the variable is really "created" for the linker).
Code:
#define tcg_qemu_tb_exec(tb_ptr) ((long REGPARM (*)(void *))code_gen_prologue)(tb_ptr)
Here is a nasty macrodefinition which simply calls machine code held by code_gen_prologie, passing there some parameter (tb_ptr).
Summary:
code_gen_prologue is a pure machine code containing some function, if you want to call this function you need to use the macro.