[libunwind] unw_* alias fixes for ELF and Mach-O

Rename the CMake option, LIBUNWIND_HERMETIC_STATIC_LIBRARY, to
LIBUNWIND_HIDE_SYMBOLS. Rename the C macro define,
_LIBUNWIND_DISABLE_VISIBILITY_ANNOTATIONS, to _LIBUNWIND_HIDE_SYMBOLS,
because now the macro adds a .hidden directive rather than merely
suppress visibility annotations.

For ELF, when LIBUNWIND_HIDE_SYMBOLS is enabled, mark unw_getcontext as
hidden. This symbol is the only one defined using src/assembly.h's
WEAK_ALIAS macro. Other unw_* weak aliases are defined in C++ and are
already hidden.

Mach-O doesn't support weak aliases, so remove .weak_reference and
weak_import. When LIBUNWIND_HIDE_SYMBOLS is enabled, output
.private_extern for the unw_* aliases.

In assembly.h, add missing SYMBOL_NAME macro invocations, which are
used to prefix symbol names with '_' on some targets.

Fixes PR46709.

Reviewed By: #libunwind, phosek, compnerd, steven_wu

Differential Revision: https://reviews.llvm.org/D93003