classes-recipe: add cython class

Recipes that use Cython typically also do some bespoke fixup.  Add a
class to centralise the logic:

- Set CYTHON_PREFIX_MAP to stop build paths appearing in generated
  objects
- Strip "Cython Metadata" blocks from generated code that ends up in the
  -src package

(From OE-Core rev: 9752da112b618362d2fe1b61c8939b8410e98553)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes-recipe/cython.bbclass

@@ -0,0 +1,11 @@
+DEPENDS:append = " python3-cython-native"
+
+# Remap the build paths that appear in generated .c code
+export CYTHON_PREFIX_MAP = "${S}=${TARGET_DBGSRC_DIR} ${B}=${TARGET_DBGSRC_DIR}"
+
+do_compile[postfuncs] = "strip_cython_metadata"
+strip_cython_metadata() {
+    # Remove the Cython Metadata headers that we don't need after the build, and
+    # may contain build paths.
+    find ${S} -name "*.c" -print0 | xargs -0 sed -i -e "/BEGIN: Cython Metadata/,/END: Cython Metadata/d"
+}