bfd: Avoid signed overflow for new_size adjustment

bfd: Avoid signed overflow for new_size adjustment

When bfd_size_type is unsigned 64-bit integer and sizeof is unsigned
32-bit integer, subtraction in

*new_size += sizeof (Elf32_External_Chdr) - sizeof (Elf64_External_Chdr);

will overflow.  Use

*new_size -= sizeof (Elf64_External_Chdr) - sizeof (Elf32_External_Chdr);

to avoid overflow.

	PR binutils/29860
	* compress.c (bfd_convert_section_setup): Avoid signed overflow
	for new_size adjustment.