aom_ports.cmake: don't add x86_abi_support.asm to source list

this avoids compilation failures when projects enable ASM language
support in project(). this file was never included in libaom_srcs.*

Bug: aomedia:3201
Change-Id: Ifc11ea9d1ef568c8695c0b3e305030d686b8ab9f
(cherry picked from commit 07deb29e3094798544ffd055c1d85929c0949b31)

aom_ports/aom_ports.cmake

diff --git a/aom_ports/aom_ports.cmake b/aom_ports/aom_ports.cmake
index e02157a..5e553ae 100644
--- a/aom_ports/aom_ports.cmake
+++ b/aom_ports/aom_ports.cmake
@@ -62,12 +62,12 @@
   endif()
 
   if(aom_ports_has_symbols)
+    # Note AOM_PORTS_INCLUDES_X86 are not added to the aom_ports target to avoid
+    # compilation issues in projects that enable ASM language support in
+    # project(). These sources were never included in libaom_srcs.*; if it
+    # becomes necessary for a particular generator another method should be
+    # used.
     target_sources(aom_ports PRIVATE ${AOM_PORTS_INCLUDES})
-
-    if("${AOM_TARGET_CPU}" STREQUAL "x86_64")
-      target_sources(aom_ports PRIVATE ${AOM_PORTS_INCLUDES_X86})
-    endif()
-
     set(AOM_LIB_TARGETS ${AOM_LIB_TARGETS} PARENT_SCOPE)
   else()
     target_sources(aom PRIVATE ${AOM_PORTS_INCLUDES})