bsps/sparc: Fix BSS initialization

Use __bss_start available via %g2 to clear the BSS section.  The usage
of _edata resulted in a copy of [_edata, __bss_start) from ROM to RAM
and then a clear to zero of this area.

Clear now only [__bss_start, _end).

c/src/lib/libbsp/sparc/shared/start/start.S

@@ -344,8 +344,6 @@ copy_data:
         /* clear the bss */
 1:
 
-        sethi %hi(_edata),%g2
-        or    %g2,%lo(_edata),%g2       ! g2 = start of bss
         sethi %hi(_end),%g3
         or    %g3,%lo(_end),%g3         ! g3 = end of bss
         mov   %g0,%g1                   ! so std has two zeros