fp-test: pick TARGET_ARM to get its specialization

fp-test: pick TARGET_ARM to get its specialization

This gets rid of the muladd errors due to not raising the invalid flag.

- Before:
Errors found in f64_mulAdd, rounding near_even, tininess before rounding:
+000.0000000000000  +7FF.0000000000000  +7FF.FFFFFFFFFFFFF
        => +7FF.FFFFFFFFFFFFF .....  expected -7FF.FFFFFFFFFFFFF v....
[...]

- After:
In 6133248 tests, no errors found in f64_mulAdd, rounding near_even, tininess before rounding.
[...]

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>