[ELF] --icf: do not fold preemptible symbols
Fixes PR44124.
A preemptible symbol may refer to a different definition at runtime.
When comparing a pair of relocations, if they refer to different
symbols, and either symbol is preemptible, the two containing sections
should be considered different.
gold has a similar rule https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=
ce97fa81e0c46d216b80b143ad8c02fff6906fef
Reviewed By: grimar
Differential Revision: https://reviews.llvm.org/D71163