This CL clarifies the macros used by the external memory and semaphore
implementations in the following way:

- Replace SWIFTSHADER_EXTERNAL_MEMORY_LINUX_MEMFD with
  SWIFTSHADER_EXTERNAL_OPAQUE_FD, and do the same for semaphore.

  This is driven by the fact that there is nothing specific to Linux
  in the VK_KHR_external_{memory,semaphore}_fd extensions. The fact
  that Linux and Android use a memfd-backed region is an implementation
  detail (that is now only captured in VkDeviceMemoryExternalLinux.h).

  This also opens the door to an OSX implementation of the extension
  that would use a Mach-based shared memory regions instead.

- Remove SWIFTSHADER_EXTERNAL_SEMAPHORE_ZIRCON_EVENT, since the
  VK_FUCHSIA_external_semaphore extension is platform-specific, and
  replace all checks against VK_USE_PLATFORM_FUCHSIA instead.

NOTE: This shall not change the generated code at all, for any
platform.

Bug: b/143122483
Bug: b/140421726
Change-Id: If3dffef15df2bf3c39b7860ed401af5a0b95e4e9
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38489Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: David Turner <digit@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>