Add random number generator and implement the darn instruction

This adds a true random number generator for the Xilinx FPGAs which
uses a set of chaotic ring oscillators to generate random bits and
then passes them through a Linear Hybrid Cellular Automaton (LHCA) to
remove bias, as described in "High Speed True Random Number Generators
in Xilinx FPGAs" by Catalin Baetoniu of Xilinx Inc., in:

https://pdfs.semanticscholar.org/83ac/9e9c1bb3dad5180654984604c8d5d8137412.pdf

This requires adding a .xdc file to tell vivado that the combinatorial
loops that form the ring oscillators are intentional.  The same
code should work on other FPGAs as well if their tools can be told to
accept the combinatorial loops.

For simulation, the random.vhdl module gets compiled in, which uses
the pseudorand() function to generate random numbers.

Synthesis using yosys uses nonrandom.vhdl, which always signals an
error, causing darn to return 0xffff_ffff_ffff_ffff.

This adds an implementation of the darn instruction.  Darn can return
either raw or conditioned random numbers.  On Xilinx FPGAs, reading a
raw random number gives the output of the ring oscillators, and
reading a conditioned random number gives the output of the LHCA.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

Makefile

@@ -58,7 +58,8 @@ uart_files = $(wildcard uart16550/*.v)
 
 soc_sim_files = $(soc_files) sim_console.vhdl sim_pp_uart.vhdl sim_bram_helpers.vhdl \
 	sim_bram.vhdl sim_jtag_socket.vhdl sim_jtag.vhdl dmi_dtm_xilinx.vhdl \
-	sim_16550_uart.vhdl
+	sim_16550_uart.vhdl \
+	random.vhdl glibc_random.vhdl glibc_random_helpers.vhdl
 
 soc_sim_c_files = sim_vhpi_c.c sim_bram_helpers_c.c sim_console_c.c \
 	sim_jtag_socket_c.c
@@ -177,7 +178,8 @@ toplevel=fpga/top-generic.vhdl
 dmi_dtm=dmi_dtm_dummy.vhdl
 
 fpga_files = $(core_files) $(soc_files) fpga/soc_reset.vhdl \
-	fpga/pp_fifo.vhd fpga/pp_soc_uart.vhd fpga/main_bram.vhdl
+	fpga/pp_fifo.vhd fpga/pp_soc_uart.vhd fpga/main_bram.vhdl \
+	nonrandom.vhdl
 
 synth_files = $(core_files) $(soc_files) $(fpga_files) $(clkgen) $(toplevel) $(dmi_dtm)