libgcc_s: expose __divmoddi4 on i386

GCC has used this for some time (since 7.0) and apparently we were
getting away with using the hidden symbol, but when linking with
--no-undefined-version we get an error unless it's properly exported.
(For anyone who wonders at the assymetry, __udivmoddi4 is indeed much
older and was introduced with GCC 3.0.)

MFC after: 3 days
Reviewed by: dim
Differential Revision: https://reviews.freebsd.org/D44878

Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Export various 128 bit long double functions from libgcc_s.so.1

These were already compiled for some time on aarch64 and riscv, by
including lib/libcompiler_rt/Makefile.inc, but never exported in the
shared library. Since gcc exports these under version GCC_4.6.0, we do
the same.

This review should replace D11482 for now. For e.g. amd64 more work is
still to be done, as compiler-rt does not seem to support 128 bit long
double math for that architecture.

Reviewed by: cem
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28690

Fix llvm-libunwind userspace build on ARM

GCC's libgcc exports a few ARM-specific symbols for ARM EABI, AEABI, or
EHABI or whatever it's called. Export the same ones from LLVM-libunwind's
libgcc_s, on ARM. As part of this, convert libgcc_s from a direct
Version.map to one constructed from component Symbol.map files. This allows
the ARM-specific Symbol.map to be included only on ARM.

Fix ARM-only oddities in struct name/aliases in LLVM-libunwind to match
non-ARM definitions and ARM-specific expectations in libcxxrt /
libcompiler_rt.

No functional change intended for non-ARM architectures.

This commit does not actually flip the switch for ARM defaults from libgcc
to llvm-libunwind, but makes it possible (to compile, anyway).