MxM example measured in ns to obtain a better reference on fast GPUs

[jjfumero]

Description

Minor fix to measure all timers in ns and then convert to seconds instead of measuring in milliseconds.

Problem description

n/a.

Backend/s tested

Mark the backends affected by this PR.

• [X] OpenCL
• [X] PTX
• [X] SPIRV

OS tested

Mark the OS where this PR is tested.

• [X] Linux
• [ ] OSx
• [ ] Windows

Did you check on FPGAs?

If it is applicable, check your changes on FPGAs.

• [ ] Yes
• [X] No

How to test the new patch?

make
tornado -m tornado.examples/uk.ac.manchester.tornado.examples.compute.MatrixMultiplication2D