libdw: Add Nim language and dwarf_srclang tests

libdw: Add Nim language and dwarf_srclang tests

DW_LANG_Nim and DW_LNAME_Nim were added to the DWARF languages.

While adding them, and the default lower bounds, I noticed DW_LANG_V
and DW_LANG_Algol68 where missing in srclang_to_language an internal
helper function.

Testing through the public api is not that easy since you need a
Dwarf_Die with the right attributes. So this patch adds a way to
compile an individual source file with an optional main function that
can directly access the internal/static functions.

Note that it is almost generic. But even though using .SECONDEXPANSION
I couldn't figure out how to create the equivalent of a rule starting
with %_check$(EXEEXT) target. So for now the rule has to repeated for
every new _check TEST. And there needs to be a line to tell make dist
to not expect the fake source: nodist_src_check_SOURCES = src_check.c

The new test pointed out that there were a few more bugs with
DW_LANG_Dylan and DW_LNAME_Mojo. Also fix those.

	* libdw/dwarf.h: Add DW_LANG_Nim and DW_LNAME_Nim.
	* libdw/Makefile.am: Add check_PROGRAMS and TESTS for
	dwarf_srclang_check.
	* libdw/dwarf_default_lower_bound.c
        (dwarf_default_lower_bound): Add DW_LANG_Nim.
	(dwarf_language_lower_bound): Add DW_LNAME_Nim.
	* libdw/dwarf_srclang.c (srclang_to_language): Handle
	DW_LANG_Dylan, DW_LANG_V, DW_LANG_Algol68 and DW_LANG_Nim.
	(language_to_srclang): Fix DW_LNAME_Mojo. Add DW_LNAME_Nim.
	(test_lang): New function guarded by MAIN_CHECK.
	(main): Likewise.

Signed-off-by: Mark Wielaard <mark@klomp.org>