This makes xil/sleep.h a stub to prevent multiple differing definitions
of sleep functions from toolchain and local headers. The non-standard
sleep definitions were not in use and can be added later if needed.
This support code is necessary for many Xilinx-provided bare metal device
drivers supported on ARM, AArch64, and MicroBlaze platforms. Support for
all of these architectures is kept under bsps/include due to multiple
architecture variants being supported which requires complex logic in
the build system. The imported files are and should be able to remain
unmodified. Import information is kept in bsps/shared/xil/VERSION.
