readelf: report DF_1_PIE as "Position-Independent Executable"

I finally found time to teach readelf to identify PIEs in the file
header display and program header display.  So in place of
"DYN (Shared object file)" which isn't completely true, show
"DYN (Position-Independent Executable file)".

It requires a little bit of untangling code in readelf due to
process_program_headers setting up dynamic_addr and dynamic_size,
needed to scan .dynamic for the DT_FLAGS_1 entry, and
process_program_headers itself wanting to display the file type in
some cases.  At first I modified process_program_header using a
"probe" parameter similar to get_section_headers in order to inhibit
output, but decided it was cleaner to separate out
locate_dynamic_sections.

binutils/
	* readelf.c (locate_dynamic_section, is_pie): New functions.
	(get_file_type): Replace e_type parameter with filedata.  Call
	is_pie for ET_DYN.  Update all callers.
	(process_program_headers): Use local variables dynamic_addr and
	dynamic_size, updating filedata on exit from function.  Set
	dynamic_size of 1 to indicate no dynamic section or segment.
	Update tests of dynamic_size throughout.
	* testsuite/binutils-all/x86-64/pr27708.dump: Update expected output.
ld/
	* testsuite/ld-pie/vaddr-0.d: Update expected output.
gdb/
	* testsuite/lib/gdb.exp (exec_is_pie): Match new PIE readelf output.

binutils/testsuite/binutils-all/x86-64/pr27708.dump

@@ -1,5 +1,5 @@
 
-Elf file type is DYN (Shared object file)
+Elf file type is DYN (Position-Independent Executable file)
 Entry point 0x5f0
 There are 11 program headers, starting at offset 64