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
diff --git a/linker/linker_phdr.h b/linker/linker_phdr.h
index f774b5d..db15ac9 100644
--- a/linker/linker_phdr.h
+++ b/linker/linker_phdr.h
@@ -90,7 +90,7 @@
 int phdr_table_protect_gnu_relro(const Elf_Phdr* phdr_table, size_t phdr_count, Elf_Addr load_bias);
 
 
-#ifdef ANDROID_ARM_LINKER
+#if defined(__arm__)
 int phdr_table_get_arm_exidx(const Elf_Phdr* phdr_table, size_t phdr_count, Elf_Addr load_bias,
                              Elf_Addr** arm_exidx, unsigned* arm_exidix_count);
 #endif