[arm] Update FP16 tests

My recent assemble_real patch (r275873) meant that we now output
negative FP16 constants in the same way as we'd output an integer
subreg of them.  This patch updates gcc.target/arm/fp16-* accordingly.

2019-09-26  Richard Sandiford  <richard.sandiford@arm.com>

gcc/testsuite/
	* gcc.target/arm/fp16-compile-alt-3.c: Expect (__fp16) -2.0
	to be written as a negative short rather than a positive one.
	* gcc.target/arm/fp16-compile-ieee-3.c: Likewise.

From-SVN: r276145