Add and use constants for the Android API levels.

Test: bionic tests still pass
Change-Id: If1b619cfc9db1d3f5d91f14f2ace71058dca0c4a
diff --git a/linker/linker_phdr.cpp b/linker/linker_phdr.cpp
index 5b0ee49..1b18450 100644
--- a/linker/linker_phdr.cpp
+++ b/linker/linker_phdr.cpp
@@ -608,7 +608,7 @@
       int prot = PFLAGS_TO_PROT(phdr->p_flags);
       if ((prot & (PROT_EXEC | PROT_WRITE)) == (PROT_EXEC | PROT_WRITE)) {
         // W + E PT_LOAD segments are not allowed in O.
-        if (get_application_target_sdk_version() > 25) {
+        if (get_application_target_sdk_version() >= __ANDROID_API_O__) {
           DL_ERR_AND_LOG("\"%s\": W + E load segments are not allowed", name_.c_str());
           return false;
         }