Identify reproducible builds in 'objdump -p' output for PE files

These are produced by MSVC when the '/Brepro' flag is used.

To quote from the PE specification [1]:

"The presence of an entry of type IMAGE_DEBUG_TYPE_REPRO indicates the
PE file is built in a way to achieve determinism or reproducibility. If
the input does not change, the output PE file is guaranteed to be
bit-for-bit identical no matter when or where the PE is produced.
Various date/time stamp fields in the PE file are filled with part or
all the bits from a calculated hash value that uses PE file content as
input, and therefore no longer represent the actual date and time when a
PE file or related specific data within the PE is produced. The raw data
of this debug entry may be empty, or may contain a calculated hash value
preceded by a four-byte value that represents the hash value length."

[1] https://docs.microsoft.com/en-us/windows/win32/debug/pe-format

bfd/ChangeLog:

2020-01-16  Jon Turney  <jon.turney@dronecode.org.uk>

	* peXXigen.c (pe_is_repro): New function.
	(_bfd_XX_print_private_bfd_data_common): Note timestamp is
	actually a build hash if PE_IMAGE_DEBUG_TYPE_REPRO is present.