[ELF][X86] Don't create IBT .plt if there is no PLT entry

https://github.com/ClangBuiltLinux/linux/issues/1606
When GNU_PROPERTY_X86_FEATURE_1_IBT is enabled, ld.lld will create .plt output
section even if there is no PLT entry. Fix this by implementing
IBTPltSection::isNeeded instead of using the default code path (which always
returns true).

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D120600

(cherry picked from commit 9d7001eb)