ARM: replace unnecessary perl with sed and the shell $(( )) operator

You can build a kernel in a cross compiling environment that doesn't
have perl in the $PATH. Commit 429f7a06 broke that for 32 bit
ARM. Fix it.

As reported by Stephen Rothwell, it appears that the symbols can be
either part of the BSS section or absolute symbols depending on the
binutils version.  When they're an absolute symbol, the $(( ))
operator errors out and the build fails.  Fix this as well.

Fixes: 429f7a06 ("ARM: decompressor: fix BSS size calculation")
Reported-by: Rob Landley <rob@landley.net>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>