[gdb/testsuite] define jit function name via macro

[gdb/testsuite] define jit function name via macro

Replaces previous approach with patching resulting ELF binary after
loading - now that each test iteration works on a separately compiled
binary it is not necessary anymore.

Tests are still being ran without debug info to preserve original test
functionality but this change opens up the possibility to enable debug
info if needed too.

gdb/testsuite/ChangeLog:

2020-03-27  Mihails Strasuns  <mihails.strasuns@intel.com>

	* lib/jit-elf-helpers.exp: Supply -DFUNCTION_NAME macro
	  definition when compiling jit-elf-solib.co.
	* gdb.base/jit-elf-main.c: Stop patching jit function name.
	* gdb.base/jit-elf-solib.c: Use FUNCTION_NAME macro value as a
	  function name.