Clean up linker architecture macros.

We don't need our own architecture macros; the standard ones will do.

This patch also fixes some __x86_64__ tests to be USE_RELA tests instead,
because they're not actually x86_64-specific.

I've cleaned up architecture-specific code slightly so where possible
all the code corresponding to a particular architecture is together.

This patch also fixes a bug in LP64 DT_PLTGOT handling, which should be
an error rather than falling through into DT_DEBUG! There was another #ifdef
bug where we'd only report unexpected DT_ entries on MIPS.

Change-Id: Id1d04e372611f641c1aa278a18e379f28af9eaf5

linker/debugger.cpp

@@ -235,7 +235,7 @@ void debuggerd_signal_handler(int n, siginfo_t* info, void*) {
         case SIGABRT:
         case SIGFPE:
         case SIGPIPE:
-#ifdef SIGSTKFLT
+#if defined(SIGSTKFLT)
         case SIGSTKFLT:
 #endif
             (void) tgkill(getpid(), gettid(), n);