xtensa: replace xtensa-specific _f{data,text} by _s{data,text}

commit a2d063ac ("extable, core_kernel_data(): Make sure all archs
define _sdata") missed xtensa.  Xtensa does have a start of data marker,
but calls it _fdata, causing

    kernel/built-in.o:(.text+0x964): undefined reference to `_sdata'

_stext was already defined, but it was duplicated by _fdata.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Chris Zankel <chris@zankel.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>