Adding options to regressions and instruction generator

Created by: XavierAubert

Added a way to customize the array supported_isa inside the file riscv_core_settings.sv using compile-time macros. This will allow one unique file to add RV32F, RV32ZFinx and RV32X extensions only when needed.

Adding this feature implied also a way to specify which riscv-dv configuration is to be used, hence the addition of the makefile variable RISCVDV_CFG. As it is the case for the other CFG variable, the RISCVDV_CFG variable must be set to the name of the cfg .yaml file inside the test folder that will be load. To be backward-compatible and if not specified, the makefile will read the default "corev-dv.yaml" file.

I added a support to this feature for shell, questa and vmanager regressions.

I added the build.cfg support from commit 441deba1 to questa and vmanager regressions