add DWFL_ASSERT and DWARF_ASSERT to help diagnosis

add DWFL_ASSERT and DWARF_ASSERT to help diagnosis

Semantic errors thrown from dwfl_assert() and dwarf_assert() lacked any
positional information to help track down where the assertion failed. We
create two new macros, DWFL_ASSERT and DWARF_ASSERT, which carry down
the __FILE__ and __LINE__ information so that the semantic_error created
contains that information, which can be printed out using -vv.

Before:

semantic error: libdwfl failure (asserting!): no error

After:

semantic error: libdwfl failure (asserting!): no error
   thrown from: ../systemtap/tapsets.cxx:7183