PowerPC: Don't generate unused section symbols

PowerPC version of git commit d1bcae83.

bfd/
	* elf32-ppc.c: Delete outdated comment.
	(TARGET_KEEP_UNUSED_SECTION_SYMBOLS): Define.
	* elf64-ppc.c (TARGET_KEEP_UNUSED_SECTION_SYMBOLS): Define.
gas/
	* testsuite/gas/ppc/power4.d: Adjust for removal of section sym.
	* testsuite/gas/ppc/test1elf32.d: Likewise.
	* testsuite/gas/ppc/test1elf64.d: Likewise.
ld/
	* testsuite/ld-powerpc/relbrlt.s: Make symbols global.
	* testsuite/ld-powerpc/relbrlt.d: Adjust to suit.
	* testsuite/ld-powerpc/tlsget.d: Adjust for reordered stubs.
	* testsuite/ld-powerpc/tlsget.wf: Likewise.
	* testsuite/ld-powerpc/tlsget2.d: Likewise.
	* testsuite/ld-powerpc/tlsget2.wf: Likewise.
	* testsuite/ld-powerpc/tlsexe.r: Adjust for removed section syms.
	* testsuite/ld-powerpc/tlsexe32.r: Likewise.
	* testsuite/ld-powerpc/tlsexe32no.r: Likewise.
	* testsuite/ld-powerpc/tlsexeno.r: Likewise.
	* testsuite/ld-powerpc/tlsexenors.r: Likewise.
	* testsuite/ld-powerpc/tlsexers.r: Likewise.
	* testsuite/ld-powerpc/tlsexetoc.r: Likewise.
	* testsuite/ld-powerpc/tlsexetocrs.r: Likewise.
	* testsuite/ld-powerpc/tlsso.r: Likewise.
	* testsuite/ld-powerpc/tlsso32.r: Likewise.
	* testsuite/ld-powerpc/tlstocso.r: Likewise.