[SYCL] Static linking support in Level Zero plugin (#5266)

The Level Zero driver now has "static linking" support, the ability to
create a single Level Zero module from a list of SPIR-V modules.  Use
this feature to implement `piProgramLink`, replacing the previous
implementation that relied on "dynamic linking" of Level Zero modules.
There are two main improvements:

* Static linking provides more optimizations because the compiler sees
  the SPIR-V for all modules, which enables cross-module optimizations.

* The previous implementation was mostly using mock driver APIs, and it
  didn't work in the general case when `piProgramLink` tried to link
  more than one input module together.

This is mostly an infrastructure improvement, which does not provide
any new features or bug fixes.  Since there is no Level Zero interop
API that can create a `kernel_bundle` in `object` or `input` state,
there is no need currently to link multiple Level Zero modules together
with `piProgramLink`.

However, this commit provides the infrastructure we need to enable
online linking of the "device library" into device code.  That feature
will be enabled in a future commit.


Note that a temporary environment variable has also been added
(`SYCL_FORCE_LINK=[0|1]`), which was added to help test this change.
Defining this variable to `1` forces the runtime to call
`piProgramCompile` / `piProgramLink` instead of `piProgramBuild` even
if the program is created from a single SPIR-V module.  This is usually
safe, but it sometimes results in false errors, so only enable this
variable if you know what you are doing.  We expect support for this
environment variable to removed soon.

Author: gmlueck

sycl/include/CL/sycl/detail/pi.h

@@ -114,6 +114,7 @@ typedef enum {
   PI_INVALID_IMAGE_FORMAT_DESCRIPTOR = CL_INVALID_IMAGE_FORMAT_DESCRIPTOR,
   PI_IMAGE_FORMAT_NOT_SUPPORTED = CL_IMAGE_FORMAT_NOT_SUPPORTED,
   PI_MEM_OBJECT_ALLOCATION_FAILURE = CL_MEM_OBJECT_ALLOCATION_FAILURE,
+  PI_LINK_PROGRAM_FAILURE = CL_LINK_PROGRAM_FAILURE,
   PI_FUNCTION_ADDRESS_IS_NOT_AVAILABLE =
       -998, ///< PI_FUNCTION_ADDRESS_IS_NOT_AVAILABLE indicates a fallback
             ///< method determines the function exists but its address cannot be