[ELF] Move R_*_IRELATIVE from .rel[a].plt to .rel[a].dyn unless --pack-dyn-relocs=android[+relr]

An R_*_IRELATIVE represents the address of a STT_GNU_IFUNC symbol
(redirected at runtime) which is non-preemptable and is not associated
with a canonical PLT (associated with a symbol with a section index of
SHN_UNDEF but a non-zero st_value).

.rel[a].plt [DT_JMPREL, DT_JMPREL+DT_JMPRELSZ) contains relocations that
can be lazily resolved. R_*_IRELATIVE are always eagerly resolved, so
conceptually they do not belong to .rela.plt. "iplt" is mostly a misnomer.

glibc powerpc and powerpc64 do not resolve R_*_IRELATIVE if they are in .rela.plt.

    // a.o - synthesized PLT call stub has an R_*_IRELATIVE
    void ifunc(); int main() { ifunc(); }
    // b.o
    static void real() {}
    asm (".type ifunc, %gnu_indirect_function");
    void *ifunc() { return ℜ }

The lld-linked executable crashes. ld.bfd places R_*_IRELATIVE in
.rela.dyn and the executable works.

glibc i386, x86_64, and aarch64 have logic
(glibc/sysdeps/*/dl-machine.h:elf_machine_lazy_rel) to eagerly resolve
R_*_IRELATIVE in .rel[a].plt so the lld-linked executable works.

Move R_*_IRELATIVE from .rel[a].plt to .rel[a].dyn to fix the crashes on
glibc powerpc/powerpc64. This also helps simplifying ifunc
implementation in FreeBSD rtld-elf powerpc64.

If --pack-dyn-relocs=android[+relr] is specified, the Android packed
dynamic relocation format is used for .rela.dyn. We cannot name
in.relaIplt ".rela.dyn" because the output section will have mixed
formats. This can be improved in the future.

Reviewed By: pcc

Differential Revision: https://reviews.llvm.org/D65651

llvm-svn: 367745