[GOLD] Add --secure-plt option for ppc32

[GOLD] Add --secure-plt option for ppc32

Added just to accept, and ignore.  gcc since 2015-10-21, when
configured with --enable-secureplt passes this option to the linker.
As powerpc gold cannot link --bss-plt code successfully, gold needs to
accept the option or the gcc specs file needs to be changed.

The patch also make gold detect --bss-plt code and error out rather
than producing a binary that crashes.

	* options.h: Add --secure-plt option.
	* powerpc.cc (Target_powerpc::Scan::local): Detect and error
	on -fPIC -mbss-plt code.
	(Target_powerpc::Scan::global): Likewise.