Don't strip section defining _SDA_BASE_

Don't strip section defining _SDA_BASE_

Prior to 93d1b056 _SDA_BASE_ used to be defined in a linker script
output section.  Now _SDA_BASE_ is defined in an input section that is
subject to being stripped.  If the section is stripped we don't output
the symbol, which results in --emit-relocs trying to emit relocs with
dangling references to _SDA_BASE_.

	* elf32-ppc.c (ppc_elf_size_dynamic_sections): Arrange to keep
	.sdata/.sdata2 when _SDA_BASE_/_SDA2_BASE_ should be output
	for --emit-relocs.