Move modification of bfd abs and und back to gas

In commit f592407e4d75 I deleted gas' obj_sec_set_private_data, and
instead put the gas modification of bfd's *ABS* and *UND* sections in
bfd_make_section_old_way.  More recently in commit 8b5a21249537 I made
tekhex symbol creation use bfd_make_section_old_way for symbol
sections.  After that we saw numerous non-repeatable oss-fuzz reports
of accesses to freed memory involving relocation symbols.  I think
what is happening is:

A tekhex testcase with an absolute symbol is run through the tool,
modifying bfd_abs_section.symbol to point to a symbol on the bfd's
objalloc memory.  On closing that bfd bfd_abs_section.symbol points to
freed memory.

A second testcase is run through the tool with some access to the
*ABS* symbol.  This triggers the invalid memory access.

The same thing could happen if a user runs objdump or nm with two
files on the command line, the first being a tekhex file with absolute
symbols, or if ld is given tekhex input among other files.  Clearly,
it's a bad idea to modify the *ABS* or *UND* sections for input files.

bfd/
	* section.c (bfd_make_section_old_way): Don't call
	_new_section_hook for standard abs, com, und and ind sections.
gas/
	* as.c (bfd_std_section_init): New function.
	(perform_an_assembly_pass): Move section initialisation to..
	(gas_init): ..here.  Use bfd_std_section_init.

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