Share proc get_var_address

Share proc get_var_address

This patch moves proc get_var_address into lib/gdb.exp, and remove the
duplicate copy in gdb.base/*.exp files.

gdb/testsuite:

2016-10-13  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/code_elim.exp (get_var_address): Remove.
	* gdb.base/relocate.exp: Likewise.
	* gdb.base/shreloc.exp: Likewise.
	* lib/gdb.exp (get_var_address): New.