Move gdb support functions to a separate file

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

Change-Id: I96c6592a7055715b18f1137367470fe80987263f
diff --git a/linker/Android.mk b/linker/Android.mk
index b940690..c37d6db 100644
--- a/linker/Android.mk
+++ b/linker/Android.mk
@@ -23,6 +23,7 @@
     dlfcn.cpp \
     linker.cpp \
     linker_block_allocator.cpp \
+    linker_gdb_support.cpp \
     linker_libc_support.c \
     linker_mapped_file_fragment.cpp \
     linker_phdr.cpp \