Add support for "orig_eax" pseudo register on Linux/x86.

Add support for "orig_eax" pseudo register on Linux/x86.
* i386-linux-tdep.c: Include "inferior.h".
(i386_linux_register_name, i386_linux_register_byte,
i386_linux_register_raw_size): New functions.
(i386_linux_write_pc): New function.
* config/i386/tm-linux.h (I386_LINUX_ORIG_EAX_REGNUM): New define.
(NUM_REGS, MAX_NUM_REGS, REGISTER_BYTES, REGISTER_NAME,
REGISTER_BYTE, REGISTER_RAW_SIZE): Define to deal with additional
register.
(i386_linux_register_name, i386_linux_register_byte,
i386_linux_register_raw_size): New prototypes.
(TARGET_WRITE_PC): New define.
(i386_linux_write_pc): New prototype.