Makefile: remove unneeded -s switch on compiling BIOS files

Stripping has no effect on object files, so having "-s -c" on the
command line makes no sense.
In fact clang complains about it and aborts with an error, so lets
just remove the unneeded "-s" switch here.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>