[xstormy16] Update xstormy16_rtx_costs.

This patch provides an improved rtx_costs target hook on xstormy16.
The current implementation has the unfortunate property that it claims
that zero_extendhisi2 is very cheap, even though the machine description
doesn't provide that instruction/pattern.  Doh!  Rewriting the
xstormy16_rtx_costs function has additional benefits, including
making more use of the (short) "mul" instruction when optimizing
for size with -Os.

2023-04-23  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/stormy16/stormy16.cc (xstormy16_rtx_costs): Rewrite to
	provide reasonable values for common arithmetic operations and
	immediate operands (in several machine modes).

gcc/testsuite/ChangeLog
	* gcc.target/xstormy16/mulhi.c: New test case.

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