[ELF] Allow discarding of .rela.plt

When linking the linux kernel on ppc64le

ld.lld -EL -m elf64lppc -Bstatic --orphan-handling=warn --build-id -o
.tmp_vmlinux1 -T ./arch/powerpc/kernel/vmlinux.lds --whole-archive
built-in.a --no-whole-archive --start-group lib/lib.a --end-group
ld.lld: error: discarding .rela.plt section is not allowed

The linker script discards with the following matches
*(.glink .iplt .plt .rela* .comment)

Differential Revision: https://reviews.llvm.org/D54871

llvm-svn: 348258