xtensa: Minor fix for FP constant synthesis

This patch fixes an non-fatal issue about negative constant values derived
from FP constant synthesis on hosts whose 'long' is wider than 'int32_t'.

And also replaces the dedicated code in FP constant synthesis split
pattern with the appropriate existing function call.

gcc/ChangeLog:

	* config/xtensa/xtensa.md:
	In FP constant synthesis split pattern, subcontract to
	avoid_constant_pool_reference() as in the case of integer,
	because it can handle well too.  And cast to int32_t before
	calling xtensa_constantsynth() in order to ignore upper 32-bit.

gcc/testsuite/ChangeLog:

	* gcc.target/xtensa/constsynth_double.c:
	Modify in order to catch the issue.