Check for 'U' extension before accessing 'mcounteren' CSR

On 64-bit Rocket with 'DefaultFPGAConfig' (using 'WithNSmallCores'),
the 'U' extension is not supported, and accessing 'mcounteren' would
trigger an 'Illegal Instruction' trap.

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>

machine/minit.c

@@ -30,6 +30,7 @@ static void mstatus_init()
   // Enable user/supervisor use of perf counters
   if (supports_extension('S'))
     write_csr(scounteren, -1);
+  if (supports_extension('U'))
     write_csr(mcounteren, -1);
 
   // Enable software interrupts