i386: fix compilation warning

Some gcc versions complain about assigning long* to uint32_t* on i386.

* syscall.c [I386] (i386_esp_ptr): Change type from uint32_t* to long*.
* linux/i386/arch_regs.h (i386_esp_ptr): Likewise.
* linux/x86_64/arch_regs.h: Do not include "i386/arch_regs.h".
(i386_esp_ptr): New prototype.
3 files changed