Use modern Verilator time handling

Created by: jeremybennett

Historically Verilator required you to implement a global function
sc_time_stamp for plain C++ models inorder to handle Verilog
$time.  Since Verilator 4.200 this is no longer needed, but
sometimes you need to be explicit about this to Verilator using a
C pre-processor define.

Files changed:

* rtl/core-v-mcu/core-v-mcu.core: Add option --CFLAGS
-DVL_TIME_CONTEXT to avoid need for sc_time_stamp.

Signed-off-by: Jeremy Bennett jeremy.bennett@embecosm.com