Reapply r354347 (by cem):

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).

(cherry picked from commit 998a72c55d958bcc1869628318477532dc6e1ffa)