link script PROVIDE bugfix

[Paul Brook]

> A bugfix for expressions like "PROVIDE(sym=ADDR(sec))", and renaming of
> a link script shell var.  I renamed because ${LARGE_SECTIONS} intervenes
> between .bss end syms and _end.  Some caution is needed if using to
> define .bss syms.

This change breaks arm-none-eabi.
elf.sc (and other scripts based on it) already contain ${OTHER_END_SYMBOLS} 
right at the end of the script, as documented at the top of that file.

The later (older) occurance overrides the one you added, so __bss_start__ ends 
up at the wrong place. 

A quick grep though emulparams/ indicates some targets are using the old 
meaning.

Paul