Add CV-X-IF extension to Spike. Improve diagnostics of extension loader.

Created by: zchamski

This PR provides a Spike extension implementing instructions for use in verifying the Core-V side of the CV-X interface in both 32b qand 64b configurations.

To use the CVXIF instructions:

• Remove the existing Spike installation located in tools/spike
• Run the Spike instllation script: sh cva6/regress/install-spike.sh
• Invoke Spike with cvxif extension: spike --extension=cvxif ...

The test program 'cva6/regress/spike/cvxif-ext-files/customext/cvxif_test.c' provides a basic test case and the instructions for checking the correct operation of the CVXIF extension.

Files changed/added:

• cva6/regress/install-spike.sh: Apply additional patches. Copy CV-X-IF files to spike source directory. Update copyright.
• cva6/regress/spike-shared-fesvr-lib.patch: Update and move to separate Spike patch directory 'cva6/regress/spike/patches'.
• cva6/regress/spike/cvxif-ext-files/customext/cvxif.cc: New.
• cva6/regress/spike/cvxif-ext-files/customext/cvxif_test.c: Ditto.
• cva6/regress/spike/cvxif-ext-files/riscv/cvxif.h: Ditto.
• cva6/regress/spike/cvxif-ext-files/riscv/cvxif_base.cc: Ditto.
• cva6/regress/spike/patches/spike-cvxif-extension.patch: Ditto.
• cva6/regress/spike/patches/spike-dlopen-diagnostics.patch: Ditto.

Signed-off-by: Zbigniew Chamski zbigniew.chamski@thalesgroup.com @Gchauvon @JeanRochCoulon @MikeOpenHWGroup : please provide your feedback on this new iteration of the PR.