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: NAlex Bennée <alex.bennee@linaro.org>
Tested-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NEmilio G. Cota <cota@braap.org>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>

tests/fp/Makefile

@@ -29,6 +29,9 @@ QEMU_INCLUDES += -I$(TF_SOURCE_DIR)
 
 # work around TARGET_* poisoning
 QEMU_CFLAGS += -DHW_POISON_H
+# define a target to match testfloat's implementation-defined choices, such as
+# whether to raise the invalid flag when dealing with NaNs in muladd.
+QEMU_CFLAGS += -DTARGET_ARM
 
 # capstone has a platform.h file that clashes with softfloat's
 QEMU_CFLAGS := $(filter-out %capstone, $(QEMU_CFLAGS))