Add XRT and ACCL support for Xilinx FPGAs

Major additions in this release are:

• add support for host codes using the XRT C++ API and HLS C/C++ device code for Xilinx FPGAs.
• add support for ACCL in b_eff, LINPACK, and PTRANS based on the XRT.

Full Changelog: v0.5.1...v0.6