gdb: move bfd_open_from_target_memory to gdb_bfd

gdb: move bfd_open_from_target_memory to gdb_bfd

This function allows to create a BFD handle using an accessible memory
range in a target memory.  It is currently contained in a JIT module but
this functionality may be of wider usefullness - for example, reading
ELF binaries contained within a core dump.

gdb/ChangeLog:
2020-12-07  Mihails Strasuns  <mihails.strasuns@intel.com>

	* jit.c (mem_bfd*, bfd_open_from_target_memory): Removed.
	* gdb_bfd.h (gdb_bfd_open_from_target_memory): New function.
	* gdb_bfd.c (mem_bfd*, gdb_bfd_open_from_target_memory): New functions.