Using mlir cmake in downstream project fails with error
```
CMake Error at D:/projs/llvm/llvm-install/lib/cmake/mlir/MLIRTargets.cmake:2537 (message):
The imported target "mlir_arm_sme_abi_stubs" references the file
"D:/projs/llvm/llvm-install/lib/mlir_arm_sme_abi_stubs.lib"
but this file does not exist. Possible reasons include:
* The file was deleted, renamed, or moved to another location.
* An install or uninstall procedure did not complete successfully.
* The installation package was faulty and contained
"D:/projs/llvm/llvm-install/lib/cmake/mlir/MLIRTargets.cmake"
but not all the files it references.
Call Stack (most recent call first):
D:/projs/llvm/llvm-install/lib/cmake/mlir/MLIRConfig.cmake:37 (include)
mlir/CMakeLists.txt:5 (find_package)
```
Windows cmake needs export libaries but it seems they are only being
generated if you have at least one exported symbol.
Add export attributes to symbols.
Not sure what the best approach to fix this (probably we should just
disable this lib on windows entirely), but it fixed things for me
locally.
Previously, we were inserting za.enable/disable intrinsics for functions
with the "arm_za" attribute (at the MLIR level), rather than using the
backend attributes. This was done to avoid a dependency on the SME ABI
functions from compiler-rt (which have only recently been implemented).
Doing things this way did have correctness issues, for example, calling
a streaming-mode function from another streaming-mode function (both
with ZA enabled) would lead to ZA being disabled after returning to the
caller (where it should still be enabled). Fixing issues like this would
require re-doing the ABI work already done in the backend within MLIR.
Instead, this patch switches to use the "arm_new_za" (backend) attribute
for enabling ZA for an MLIR function. For the integration tests, this
requires some way of linking the SME ABI functions. This is done via the
`%arm_sme_abi_shlib` lit substitution. By default, this expands to a
stub implementation of the SME ABI functions, but this can be overridden
by providing the `ARM_SME_ABI_ROUTINES_SHLIB` CMake cache variable
(pointing it at an alternative implementation). For now, the ArmSME
integration tests pass with just stubs, as we don't make use of nested
ZA-enabled calls.
A future patch may add an option to compiler-rt to build the SME
builtins into a standalone shared library to allow easily
building/testing with the actual implementation.
