fix(xcelium): handling DPI libraries

[tymonx]

Closes #548

The xrun command is just a wrapper to invoke:

• xmvhdl to compile VHDL source files
• xmvlog to compile Verilog and SystemVerilog source files
• xmelab to perform elaboration of HDL module
• xmsim to run simulation

Handling DPI libraries is only done by the xmsim command (simulation runtime):

xmsim -help | grep -i dpi
        -DPI_STACK_INT_C                    -- Lists internal C stack frames called from import function
        -SV_LIB <arg>                       -- Dynamically load a DPI Library

The elaboration stage is only used to generate header files for export/import functions/tasks:

xmelab | grep -i dpi
        -DPI_VOID_TASK                          -- Return value of export and import tasks will be VOID. 
        -DPIHEADER <arg>                        -- generate the header file for export functions/tasks
        -DPIIMPHEADER <arg>                     -- generate the header file for import functions/tasks
        -GCC_VERS <arg>                         -- Pass gcc_vers switch for DPI file compilation

DPI libraries lib<name>.so can be passed only to the simulation runtime stage:

xmsim -sv_lib <dir>/lib<name>.so

Without it, xsim (or xrun -R) will raise an error NOFDPI:

xmsim: *F,NOFDPI (<file>.sv,<line>|<column>): Function <name> not found in any of the shared object specified with -SV_LIB switch. The corresponding import DPI declaration is done in file <file>.sv at line no. <line>.

About the raised error:

xmhelp xmsim NOFDPI

xmsim/NOFDPI =
        C function declared in import DPI declaration should be present in
        shared library. Specified function is not available in any of the
        provided shared objects or default "libdpi".
        (Default severity: 'F')

        See more info at: https://support.cadence.com/apex/Coveo_CommunitySearch#ph=Xcelium&t=PlatformProductPage&oMenu=Search&cadproduct=%22Xcelium%22&q=NOFDPI

This PR:

• Fixed handling DPI libraries
• DPI libraries are passed to the simulation runtime stage xrun -R (aka xmsim)
• Removes DPI libraries from elaboration stage. They where ignored by the xrun elaboration stage
• DPI libraries were composed as list of list (dpi_lib_cmd) and concatenated with xrun arguments generating invalid arguments at the end ['-sv_lib', 'libdpi.so']
• Adds additional libdpi.so file for test fixture (and fixes legacy xcelium tests because of that...)
• Extends test_tool_xcelium to check if the xrun -R simulation command contains DPI arguments -sv_dpi libdpi.so