[libc] Support poky system

[mikhail.ramalho]

Summary

This patch adds support for yocto images, which are custom Linux-base
systems created by yocto.

$CMAKE_HOST_SYSTEM_NAME returns "poky" as the system name, but it is a
linux image, so we just replace the name with "linux", so libc can use
the correct path.

[michaelrj]

LGTM with nit

libc/cmake/modules/LLVMLibCArchitectures.cmake

167

Nit: there should be a comment explaining what poky is and why it's equivalent to linux here.

libc/cmake/modules/LLVMLibCArchitectures.cmake

else()
   message(FATAL_ERROR
           "Unsupported libc target architecture ${LIBC_TARGET_ARCHITECTURE}")
 endif()
 
 if(LIBC_TARGET_OS STREQUAL "baremetal")
   set(LIBC_TARGET_OS_IS_BAREMETAL TRUE)
 elseif(LIBC_TARGET_OS STREQUAL "linux")
   set(LIBC_TARGET_OS_IS_LINUX TRUE)
+elseif(LIBC_TARGET_OS STREQUAL "poky")
+  # poky are ustom Linux-base systems created by yocto. Since these are Linux
+  # images, we change the LIBC_TARGET_OS to linux. This define is used to
+  # include the right directories during compilation.
+  set(LIBC_TARGET_OS_IS_LINUX TRUE)
+  set(LIBC_TARGET_OS "linux")
 elseif(LIBC_TARGET_OS STREQUAL "darwin")
   set(LIBC_TARGET_OS_IS_DARWIN TRUE)
 elseif(LIBC_TARGET_OS STREQUAL "windows")
   set(LIBC_TARGET_OS_IS_WINDOWS TRUE)
 else()
   message(FATAL_ERROR
           "Unsupported libc target operating system ${LIBC_TARGET_OS}")
 endif()