Bumping version to 1.8.0
ReleaseNotes and news update.
Adding ISPC_PS4 macro support, which does the following:
 - allows "btver2" cpu name for target info.
 - disables cpu selection via command line.
 - make btver2 default cpu.
 - sets avx1-i32x4 as default target for btver2 CPU.
Cross-compiler for PS4 is targeted UNIX, not Windows - this needs to be
addressed in ispc.vcxproj file.
Use "x86_64-unknown-freebsd" tripple for PS4.
-Fix typo in ispc.cpp under ISPC_PS4
-Pass ISPC_PS4 define in compilation flags on Windows
Disable selecting arch for PS4
unistd include path
Adding PS4 specific version string