There's a handful of things that went wrong here:

* The read-only data sections were mapped to flash, which is very slow.
  I just put them in the data segment, so they end up in the scratchpad.
  This is about a 10x hit, so it's really important.
* The toolchain was an old version, which didn't have a fast memcpy
  implementation on 32-bit systems.  This is about a 2x hit.
* Some compiler flags were incorrect, including
    * -Os instead of -O3
    * Missing -mexplicit-relocs
    * Missing -DNOENUM
    * Missing -falign-functions=4
  I haven't checked how much those hurt

With this, I get

$ make software BOARD=freedom-e300-hifive1 PROGRAM=dhrystone LINK_TARGET=dhrystone
$ make upload BOARD=freedom-e300-hifive1 PROGRAM=dhrystone LINK_TARGET=dhrystone
Execution starts, 10000000 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]:    10000010
        should be:   Number_Of_Runs + 10
Ptr_Glob->
  Ptr_Comp:          -2147470264
        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:          -2147470264
        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: 1.3
Dhrystones per Second:                      714285.6

which is 1.55 DMIPS/MHz at 262 MHz.  It's still a bit slower than our
current stuff, but I don't remember what was actually in the HiFive1 so
I'm not sure what we should be getting.  I verified the clock is
accurate with a stopwatch.  I haven't bothered to go look through the
binary, but I think we're about 10 cycles off so it should be managable.