[gdb/testsuite] Fix main high_pc in nonvar-access.exp

When running gdb.dwarf2/nonvar-access.exp with board readnow, we have:
...
FAIL: gdb.dwarf2/nonvar-access.exp: print/x def_implicit_s
...
and 12 more similar failures.

I've tracked this down to the range of main being hardcoded to
[_main, _main+0x10000) in the dwarf assembly:
...
            DW_TAG_subprogram {
                {name main}
                {DW_AT_external 1 flag}
                {low_pc [gdb_target_symbol main] DW_FORM_addr}
		{high_pc [gdb_target_symbol main]+0x10000 DW_FORM_addr}
            } {
...
which overlaps with the .debug_info for the elf-init.c CU (containing
__libc_csu_init and __libc_csu_fini).

Fix this by using function_range to find the actual range of main.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-06-11  Tom de Vries  <tdevries@suse.de>

	PR testsuite/24521
	* gdb.dwarf2/nonvar-access.exp: Fix main high_pc.