sigjmp_buf and jmp_buf are incompatible types on i386

Relibc currently defines the prototype of sigsetjmp as int32_t sigsetjmp(uint64_t *, int32_t) on all platforms, while on i386, setjmp is defined as int setjmp(jmp_buf) where jmp_buf is unsigned long[6]. siglongjmp and longjmp are different in the same way. This currently causes 32-bit Redox builds to fail due to assumptions gettext makes that the first arguments of setjmp and sigsetjmp are equivalent, ultimately ending with a multiple symbol definitions error when compiling. Due to the fact that sigsetjmp isn't actually implemented on anything other than amd64 currently anyway and the way gettext macros its sigsetjmp calls, this issue can be solved for now by simply putting the prototypes for sigsetjmp and siglongjmp in signal.h behind an #ifdef __x86_64__.