[gdb/testsuite] add lib/jit-elf-helpers.exp

[gdb/testsuite] add lib/jit-elf-helpers.exp

New utility library to be used by jit-elf tests responsible for
compiling binary artifacts. In the next commit the compilation process
will become more complicated because of extra mandatory flag - keeping
it in one place will make tests less fragile.

gdb/testsuite/ChangeLog:

2020-02-18  Mihails Strasuns  <mihails.strasuns@intel.com>

	* lib/jit-elf-helpers.exp: New file.
	* gdb.base/jit-elf.exp: Updated to use jit-elf-helpers.exp.
	* gdb.base/jit-elf-so.exp: Updated to use jit-elf-helpers.exp.