Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Pass --enable-new-dtags to the linker invocation by default. If
desired, one can turn off the generation of post-ELF standard dtags by
overriding it with --disable-new-dtags after the default switch.

Immediate effect of the change is that -rpath path is now stored both
in DT_RPATH and DT_RUNPATH tags, which is the right way to provide
rpath for dynamic linker supporting DT_RUNPATH per specification.

Reviewed by: kan
MFC after: 1 month

Now that the fix for gcc PR 20218 is applied, and hidden attributes
actually work, linking of libgcc_s.so.1 on ia64 will fail with:

unwind-ia64.So(.text+0x1762): In function `_Unwind_FindEnclosingFunction':
: undefined reference to `_Unwind_FindTableEntry'
unwind-ia64.So(.text+0x1d82): In function `uw_frame_state_for':
: undefined reference to `_Unwind_FindTableEntry'
/usr/bin/ld: libgcc_s.so.1: hidden symbol `_Unwind_FindTableEntry' isn't defined

Repair this by not hiding the _Unwind_FindTableEntry symbol; on FreeBSD,
it is in libc, not in libgcc.

Silence from: current@

Apply backported fix for gcc PR 20218. This makes __attribute__
((visibility ("hidden"))) work properly, and fixes building devel/glib20
with newer binutils.

Silence from: current@

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