Add back Dhrystone port

sw/example/dhrystone/LICENSE

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+Original LICENSE from https://github.com/sifive/benchmark-dhrystone
+
+Dhrystone
+------------------------------------------------------------------------------
+There is no explicit license defined.  They were originally
+written in ADA by Reinhold P. Weicker and translated to C by Rick Richardson .
+
+The source obtained from the following site:
+https://fossies.org/linux/privat/old/dhrystone-2.1.tar.gz

sw/example/dhrystone/README.md

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+## Dhrystone Benchmark
+
+:copyright: Original sources from [https://github.com/sifive/benchmark-dhrystone](https://github.com/sifive/benchmark-dhrystone);
+ see `LICENSE` file. The origianl source code has been modified for the NEORV32 RISC-V Processor.
+
+To compile the `main.exe` executable:
+
+```bash
+neorv32/sw/example/dhrystone$ make USER_FLAGS+=-DRUN_DHRYSTONE MARCH=rv32imc_zicsr_zifencei EFFORT=-O2 clean_all exe
+...
+Memory utilization:
+   text    data     bss     dec     hex filename
+   7976       0   10448   18424    47f8 main.elf
+Compiling ../../../sw/image_gen/image_gen
+Executable (neorv32_exe.bin) size in bytes:
+7988
+```
+
+The default number of iterations is 10000. You can modify this by adding `USER_FLAGS+=-DDHRY_ITERS=2000000` to the makefile invocation.
+Dhrystone will require an IMEM size of at least 8kB and a DMEM size of about 11kB. The MTIME machine timer is used for timing benchmarking.
+Note that the Drhystone score is normalized to the original VAX machine (SiFive is giving a nice overview
+about this at https://www.sifive.com/blog/dhrystone-performance-tuning-on-the-freedom-platform):
+
+```
+VAX DMIPS/s/MHz = dhrystone_iterations / execution_cycles * clock_frequency_in_hz / 1757
+```
+
+### Exemplary Output
+
+Output generated for processor HW version [v1.9.9.2](https://github.com/stnolting/neorv32/blob/main/CHANGELOG.md).
+All results only show the integer parts.
+
+```
+NEORV32: Processor running at 100000000 Hz
+NEORV32: Executing Dhrystone (10000 iterations). This may take some time...
+
+
+Dhrystone Benchmark, Version 2.1 (Language: C)
+
+Program compiled without 'register' attribute
+
+Execution starts, 10000 runs through Dhrystone
+Execution ends
+
+Final values of the variables used in the benchmark:
+
+Int_Glob:            5
+        should be:   5
+Bool_Glob:           1
+        should be:   1
+Ch_1_Glob:           A
+        should be:   A
+Ch_2_Glob:           B
+        should be:   B
+Arr_1_Glob[8]:       7
+        should be:   7
+Arr_2_Glob[8][7]:    10010
+        should be:   Number_Of_Runs + 10
+Ptr_Glob->
+  Ptr_Comp:          2147483732
+        should be:   (implementation-dependent)
+  Discr:             0
+        should be:   0
+  Enum_Comp:         2
+        should be:   2
+  Int_Comp:          17
+        should be:   17
+  Str_Comp:          DHRYSTONE PROGRAM, SOME STRING
+        should be:   DHRYSTONE PROGRAM, SOME STRING
+Next_Ptr_Glob->
+  Ptr_Comp:          2147483732
+        should be:   (implementation-dependent), same as above
+  Discr:             0
+        should be:   0
+  Enum_Comp:         1
+        should be:   1
+  Int_Comp:          18
+        should be:   18
+  Str_Comp:          DHRYSTONE PROGRAM, SOME STRING
+        should be:   DHRYSTONE PROGRAM, SOME STRING
+Int_1_Loc:           5
+        should be:   5
+Int_2_Loc:           13
+        should be:   13
+Int_3_Loc:           7
+        should be:   7
+Enum_Loc:            1
+        should be:   1
+Str_1_Loc:           DHRYSTONE PROGRAM, 1'ST STRING
+        should be:   DHRYSTONE PROGRAM, 1'ST STRING
+Str_2_Loc:           DHRYSTONE PROGRAM, 2'ND STRING
+        should be:   DHRYSTONE PROGRAM, 2'ND STRING
+
+Microseconds for one run through Dhrystone: 13
+Dhrystones per Second:                      72939
+
+NEORV32: << DETAILED RESULTS (integer parts only) >>
+NEORV32: Total cycles:      13710151
+NEORV32: Cycles per second: 100000000
+NEORV32: Total runs:        10000
+
+NEORV32: DMIPS/s:           72939
+NEORV32: DMIPS/s/MHz:       729
+
+NEORV32: VAX DMIPS/s:       41
+NEORV32: VAX DMIPS/s/MHz:   729/1757
+```