ppc32 and "LOAD segment with RWX permissions"

When using a bss-plt we'll always trigger the RWX warning, which
disturbs gcc test results.  On the other hand, there may be reason to
want the warning when gcc is configured with --enable-secureplt.
So turning off the warning entirely for powerpc might not be the best
solution.  Instead, we'll turn off the warning whenever a bss-plt is
generated, unless the user explicitly asked for the warning.

bfd/
	* elf32-ppc.c (ppc_elf_select_plt_layout): Set
	no_warn_rwx_segments on generating a bss plt, unless explicity
	enabled by the user.  Also show the bss-plt warning when
	--warn-rwx-segments is given without --bss-plt.
include/
	* bfdlink.h (struct bfd_link_info): Add user_warn_rwx_segments.
ld/
	* lexsup.c (parse_args): Set user_warn_rwx_segments.
	* testsuite/ld-elf/elf.exp: Pass --secure-plt for powerpc to
	the rwx tests.