ISPC: Generated Headers suffix configurable with a better default

The target property `ISPC_HEADER_SUFFIX` and associated global
variable now can control the suffix used when generating the
C/C++ interoperability ISPC headers.

In addition the default suffix is now "_ispc.h" which matches the
common convention that the ISPC compiler team uses and recommends.
28 jobs for ispc_control_header_suffixes in 0 seconds (queued for 1 second)