Create output directories for perlasm.

Flattening the build seems to have changed the order of actions when
using Make and output directories for perlasm are no longer created
before Perl is run. Additionally, if the output directory doesn't exist,
the perlasm scripts seem to output to stdout instead.

Change-Id: I59b801f7347951a3b9cef2ff084b28a00b2d5a3c
Reviewed-on: https://boringssl-review.googlesource.com/31645
Reviewed-by: Adam Langley <agl@google.com>

crypto/CMakeLists.txt

@@ -62,8 +62,14 @@ if(NOT OPENSSL_NO_ASM)
 endif()
 
 function(perlasm dest src)
+  get_filename_component(dir ${dest} DIRECTORY)
+  if ("${dir}" STREQUAL "")
+    set(dir ".")
+  endif()
+
   add_custom_command(
     OUTPUT ${dest}
+    COMMAND ${CMAKE_COMMAND} -E make_directory ${dir}
     COMMAND ${PERL_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/${src} ${PERLASM_STYLE} ${PERLASM_FLAGS} ${ARGN} ${dest}
     DEPENDS
     ${src}