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.
diff --git a/syscall.c b/syscall.c
index 209542c..5e1e021 100644
--- a/syscall.c
+++ b/syscall.c
@@ -661,8 +661,7 @@
 
 #if defined(I386)
 static struct user_regs_struct i386_regs;
-/* Cast suppresses signedness warning (.esp is long, not unsigned long) */
-uint32_t *const i386_esp_ptr = (uint32_t*)&i386_regs.esp;
+long *const i386_esp_ptr = &i386_regs.esp;
 # define ARCH_REGS_FOR_GETREGS i386_regs
 #elif defined(X86_64) || defined(X32)
 /*