cpu-exec.c: Correct comment about this file and indentation cleanup

Each target uses the #define macro (in target-xxx/cpu.h) to rename
cpu_exec (cpu-exec.c) to cpu_xxx_exec, then defines its own cpu_loop
which calls cpu_xxx_exec. So basically, cpu-exec.c is not only the i386
emulator main execution loop. This patch corrects the comment of this
file and does indentation cleanup.

Signed-off-by: Chen Wei-Ren (陳韋任) <chenwj@iis.sinica.edu.tw>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>