Fix build problems caused by "_end" -> "__bss_end__" rename

Commit 44c6e659 "rename _end to __bss_end__ broke building of a large
number of systems (at least all PowerPC?):

libstubs.o: In function `app_startup':
examples/standalone/stubs.c:197: undefined reference to `__bss_end__'

The rename should not be done for the files in the
examples/standalone/ directory, as these are not using the code from
start.S, but do their own BSS clearing, and either use their own
linker scripts or the ones provided by the compilers.

Signed-off-by: Po-Yu Chuang <ratbert@faraday-tech.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>