Move gdb support functions to a separate file

Move gdb support functions and variables to
linker_gdb_support.h/cpp

Bug: http://b/27533895
Change-Id: I96c6592a7055715b18f1137367470fe80987263f
(cherry picked from commit 6b788eeff2ea0019849517e796b762ae790ca142)
diff --git a/linker/Android.mk b/linker/Android.mk
index 1620888..4a4ca5c 100644
--- a/linker/Android.mk
+++ b/linker/Android.mk
@@ -11,6 +11,7 @@
     linker_allocator.cpp \
     linker_block_allocator.cpp \
     linker_dlwarning.cpp \
+    linker_gdb_support.cpp \
     linker_libc_support.c \
     linker_mapped_file_fragment.cpp \
     linker_memory.cpp \