gdb/testsuite: modernize gdb.dwarf2/dw2-noloc.exp

gdb/testsuite: modernize gdb.dwarf2/dw2-noloc.exp

The test gdb.dwarf2/dw2-noloc.exp predates the dwarf assembler, and uses
some unreliable assumptions about where global labels get put.
Specifically, when using clang to compile the test, both labels it uses
to gauge the adresses of the main function get reshuffled to be side-by-side,
and the debug information ends up making it look like main's high pc is equal
to low pc, meaning we never enter the main function's scope, and that leads to
22 failures because the "main_*" variables are technically never in scope.

This patch modernizes the aforementioned test to use the dwarf
assembler, which removes all failures when using clang.  It also renames
the .c file to be more inline with current standard.

Approved-By: Tom Tromey <tom@tromey.com>