Signed-off-by: Nathaniel Graff <nathaniel.graff@sifive.com>

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.

Everything still defaults to the flash, but users can optionally select
the scratchpad.  Note that you have to be pretty careful about your
scratchpad programs, most of them won't fit.  For example, printf is too
big to fit in the scratchpad.

Since our loader doesn't support multiple sections anyway, there's
really no reason to have all these sections floating around.  They're
causing trouble due to misalignment so it's easiest to just group them
together so we can properly enforce alignment.

I'm hoping this will make everything more likely to align.  We can't use
the other mechanism because our loader is lazy and assumes all the
sections are contiguous.

Without these alignment directives the sections are only required to be
aligned on word-sized boundries, but we sometimes require double-word
alignment.  This allows the linker to sanely emit these sections.