fix bundled tbb build issues on macos

[kevinushey]

Summary

Fixes the RcppParallel build failure on the CRAN macOS machines
(https://www.r-project.org/nosvn/R.check/r-release-macos-arm64/RcppParallel-00install.html),
and resolves the getcontext / swapcontext / makecontext deprecation warnings there.

tbbbind link failure

oneTBB's CMake automatically searches for hwloc via pkg-config and, if found, builds the
optional tbbbind NUMA-support library. The CRAN macOS machines now have a static-only
hwloc discoverable via pkg-config, but pkg_search_module() requests the shared-link flags,
which omit the Libs.private frameworks (CoreFoundation, IOKit, OpenCL) required when
linking hwloc statically. The tbbbind_2_5 link then fails with undefined symbols.

RcppParallel doesn't use TBB's NUMA APIs (and libtbb only ever loads tbbbind lazily via
dlopen, falling back gracefully when absent), so we now pass
-DTBB_DISABLE_HWLOC_AUTOMATIC_SEARCH=1 and skip tbbbind entirely. This also makes the
build deterministic regardless of what's installed on the build machine, and avoids shipping
a libtbbbind bound to a build-machine hwloc that wouldn't exist on user machines.

ucontext deprecation warnings

The ucontext APIs used by oneTBB's coroutine implementation for resumable tasks have been
deprecated on macOS since 10.6. We now build the bundled oneTBB with
-D__TBB_RESUMABLE_TASKS_USE_THREADS=1 on macOS, selecting oneTBB's threads-based
coroutine implementation (the same code path upstream uses under sanitizers), so the
deprecated APIs are no longer referenced at all.

Testing

• Fresh R CMD INSTALL --preclean on macOS arm64 succeeds; CMake logs confirm no tbbbind
  targets are created and the define is applied to all TBB compile lines.
• nm -u libtbb.dylib shows no _getcontext / _swapcontext / _makecontext references
  (previously present).
• Installed libs are exactly libtbb.dylib, libtbbmalloc.dylib, libtbbmalloc_proxy.dylib;
  package loads and setThreadOptions() / defaultNumThreads() work.