gdb.exp: Support absolute path name args in 'prepare_for_testing' etc.

Test cases that produce source files in the build directory have not
been able to use prepare_for_testing and friends.  This was because
build_executable_from_specs unconditionally prepended the source
directory path name to its arguments.
Exploit 'prepare_for_testing' etc. for 'Dwarf::assemble'-generated files

Now that prepare_for_testing etc. can cope with absolute path names,
this can be exploited for test cases with generated source files.
This is just to simplify the code and shouldn't cause any functional
change.
Migrate dw2-ifort-parameter.exp to Dwarf::assemble

A "side effect" of the migration to Dwarf::assemble is that the DWARF
address size is now automatically adjusted to the target architecture.
The original assembler source hard-coded the DWARF address size to 4,
even on 64-bit architectures.  This address size mismatch caused a
test case failure on s390x due to a wrong result from DW_OP_deref.
Fix dw2-ifort-parameter.exp on PPC64

On PPC64, 'func' and 'main' are function descriptors and don't point
to the actual code.  Thus the usage of these symbols in the DWARF
assembler source was broken.  The patch introduces new labels
func_start and func_end for this purpose.
The value of a bignum expression is held in a single global array.  This means
that if multiple bignum values are encountered only the most recent is valid.
If such expressions are cached, eg to be emitted into a literal pool later on
in the assembly, then only one expression - the last - will be correct.  This
patch fixes the problem for the AArch64 target by caching each bignum value
locally.

	PR gas/16688
	* config/tc-aarch64.c (literal_expression): New structure.
	(literal_pool): Replace exp array with literal_expression array.
	(add_to_lit_pool): When adding a bignum cache the big value.
	(s_ltorg): When emitting a bignum initialise the global bignum
	array from the cached value.

	* gas/aarch64/litpool.s: New test case.
	* gas/aarch64/litpool.d: Expected disassembly.