Restore fixes reverted by r211607:

r209631: Use MIUtilSystemLinux on FreeBSD as well

  We should later rename this file (probably MIUtilSystemPOSIX), but
  more clean-up is still needed here, and we can wait until we better
  understand how this code may be shared between FreeBSD, Linux, and OS X.

r209632: Add stdlib.h for malloc and friends

r209633: Remove include of obsolete stropts.h header

  The header is for POSIX streams functionality, and does not exist on
  FreeBSD, OS X, or contemporary Linux distributions.

Issue reported by John Wolfe.

llvm-svn: 211620