configure: Use x86 instead of i386 for $cpu for IA32

configure: Use x86 instead of i386 for $cpu for IA32

fio internally uses "x86" for any sort of IA32, so use x86 for $cpu
as well which gets printed on ./configure.

This change should have been a part of e12f4ede in 2016.

 # grep i386 . -rI
 ./arch/arch.h:#if defined(__i386__)
 ./configure:elif check_define __i386__ ; then
 ./configure:  cpu="i386"
 ./configure:  i386|i486|i586|i686|i86pc|BePC)
 ./configure:    cpu="i386"
 ./crc/sha1.c:#if defined(__i386__) || defined(__x86_64__)
 ./crc/xxhash.c:#if defined(__ARM_FEATURE_UNALIGNED) || defined(__i386) || defined(_M_IX86) || defined(__x86_64__) || defined(_M_X64)

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>