coreaudio: split implementation by SDK version (pre-10.7 legacy path)

The current implementation uses C11 <stdatomic.h> and Grand Central
Dispatch (<dispatch/dispatch.h>), both of which require 10.6/10.7-era
SDKs and toolchains.  On Xcode 3.1 / 10.4-10.5 / PowerPC neither
header exists and the file cannot be compiled at all.

Earlier shim attempts (24157a0, 1c5c124) tried to reimplement the
C11/GCD subset on top of OSAtomic and Mach semaphores, but kept
running into header-visibility issues that are impractical to solve
portably.  Simpler approach: pick one of two full implementations at
preprocess time.

  - Pre-10.7: restore the pre-rewrite implementation (from 5b9763b)
    that uses RetroArch's slock_t/scond_t and fifo_buffer_t.  The
    audio_driver_t initializer gains a NULL write_raw slot to match
    the current struct shape; the frontend's generic resampler
    handles what the hardware-accelerated path used to.

  - 10.7+: unchanged current code path.

Selected via AvailabilityMacros.h: RARCH_COREAUDIO_LEGACY is defined
when MAC_OS_X_VERSION_10_7 is absent or MIN_REQUIRED < 10.7.  Modern
builds are bit-for-bit identical.

Author: LibretroAdmin