zlib-gabi to zstd woes

So we had a zlib-gabi .debug_info section that increased in size with
zstd, so much so that it was better to leave the section
uncompressed.  Things went horribly wrong when the section was read
again later.  The section was read again off disk using the
uncompressed size.  So you get the zlib section again with some
garbage at the end.  Fix that particular problem by setting the
section flag SEC_IN_MEMORY.  Any future read will get sec->contents.

Also, if the section is to be left uncompressed, the input
SHF_COMPRESSED flag needs to be reset otherwise objcopy will copy it
to output.

Finally, bfd_convert_section_contents needed a small update to handle
zstd compressed sections, and I've deleted bfd_cache_section_contents.

	* bfd.c (bfd_convert_section_contents): Handle zstd.
	* compress.c (bfd_compress_section_contents): When section
	contents are uncompressed set SEC_IN_MEMORY flag,
	compress_status to COMRESS_SECTION_NONE, and clear
	SHF_COMPRESSED.  Set SEC_IN_MEMORY for compressed contents.
	(bfd_get_full_section_contents): Don't check section size
	against file size when SEC_IN_MEMORY.
	(bfd_cache_section_contents): Delete function.
	* elf32-arm.c (elf32_arm_get_synthetic_symtab): Expand
	bfd_cache_section_contents here.
	* bfd-in2.h: Regenerate.