[gdb/testsuite] Fix tcl error in jit-elf-helpers.exp

[gdb/testsuite] Fix tcl error in jit-elf-helpers.exp

When running test-case jit-elf.exp with target board cc-with-gdb-index, I run
into:
...
gdb compile failed, outputs/gdb.base/jit-elf/.tmp/jit-elf-main-attach: \
  No such file or directory.
ERROR: tcl error sourcing src/gdb/testsuite/gdb.base/jit-elf.exp.
ERROR: can't read "main_basename": no such variable
    while executing
"untested "failed to compile ${main_basename}.c""
    (procedure "compile_jit_main" line 7)
...

The problem is in compile_jit_main in lib/jit-elf-helpers.exp, where we try to
emit an untested message using global variable main_basename.c.

Fixing this by declaring the variable global results in duplicate test-names,
because the same source file is compiled more than once.

Instead, fix this by using the result name in the untested message.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-12  Tom de Vries  <tdevries@suse.de>

	* lib/jit-elf-helpers.exp: Don't use undefined variables in untested
	messages.