The value of a bignum expression is held in a single global array. This means

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.