style: fix missing spaces in control statements

Result of running 'hg m5style --skip-all --fix-control -a'.

sim: Add support for generating back traces on errors

Add functionality to generate a back trace if gem5 crashes (SIGABRT or
SIGSEGV). The current implementation uses glibc's stack traversal
support if available and stubs out the call to print_backtrace()
otherwise.

atomicio: provide atomic read and write functions.

These functions keep trying to read and write until all data has been
transferred, or an error occurrs. In the case where an end of file
hasn't been reached, but all of the bytes have not been read/written,
try again. On EINTR, try again.