This CL adds a Posix-based implementation for the
VK_KHR_external_memory_fd Vulkan extension, using the SysV
shm_open() / shm_unlink() API to create the shared
memory region.

The Linux backend still uses memfd-based regions which
are easier to create, and because Android does not provide
shm_open() intentionally [1].

[1] https://android.googlesource.com/platform/ndk/+/4e159d95ebf23b5f72bb707b0cb1518ef96b3d03/docs/system/libc/SYSV-IPC.TXT

Bug: b/140419396
Tests: dEQP-VK.*
Change-Id: Ibbb23c3af59e81f76e41a0e71281f6d1a8b07c01
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41408Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Yilong Li <liyl@google.com>
Commit-Queue: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>