libc/csu: Unify INIT_RELOCS across architectures

Some architectures don't need any arguments, whilst others need auxargs,
which they get by passing in env thanks to INIT_RELOCS referencing the
local

libc/csu: Unify INIT_RELOCS across architectures

Some architectures don't need any arguments, whilst others need auxargs,
which they get by passing in env thanks to INIT_RELOCS referencing the
local variable in __libc_start1(_gcrt) by name. This is unnecessarily
confusing, fragile (one has to look at INIT_IRELOCS's definition to see
that it uses env) and duplicates code between architectures.

Instead, implement it more like rtld-elf. Each architecture provides an
ifunc_init that takes the auxargs directly, and those that don't need it
can just ignore it.

Reviewed by: kib
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D47188

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csu: move common code to libc

Why? Most trivial point, it shaves around 600 bytes from the dynamic
binaries on amd64. Less trivial, the removed code is no longer part of
the ABI, and we can ship upd

csu: move common code to libc

Why? Most trivial point, it shaves around 600 bytes from the dynamic
binaries on amd64. Less trivial, the removed code is no longer part of
the ABI, and we can ship updates to it with libc updates. Right now most
of the csu is linked into the binaries and require us to do somewhat
tricky ABI compat when it needs to change. For instance, the init_array
change would be much simpler and does not require note tagging if we
have init calling code in libc.

This could be improved more, by splitting dynamic and static
initialization. For instance, &_DYNAMIC tests can be removed then.
Such change, nonetheless, would require building libc three times.
I left this for later, after this change stabilizes, if ever.

Reviewed by: markj
Discussed with: jrtc27 (some objections, see the review), imp
Tested by: markj (aarch64)
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks
Differential revision: https://reviews.freebsd.org/D37220

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