elf: work around a gcc bug in elf_get_dynamic_info

Since commit 2f056e8a
"aarch64: define PI_STATIC_AND_HIDDEN",
building glibc with gcc-8 on aarch64 fails with

/BLD/elf/librtld.os: in function `elf_get_dynamic_info':
/SRC/elf/get-dynamic-info.h:70:(.text+0xad8): relocation truncated to
 fit: R_AARCH64_ADR_PREL_PG_HI21 against symbol `_rtld_local' defined
 in .data section in /BLD/elf/librtld.os

This is a gcc bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98618


The bug is fixed on gcc-10 and not yet backported. gcc-9 is affected,
but the issue happens to not trigger in glibc, gcc-8 and older seems
to miscompile rtld.os.

Rewriting the affected code in elf_get_dynamic_info seems to make the
issue go away on <= gcc-9.

The change makes the logic a bit clearer too (by separating the index
computation and array update) and drops an older gcc workaround (since
gcc 4.6 is no longer supported).

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>