Change SET_USR_FIELD to write to hex_new_value[HEX_REG_USR] instead
of hex_gpr[HEX_REG_USR].

Then, we need code to mark the instructions that can set implicitly
set USR
- Macros added to hex_common.py
- A_FPOP added in translate.c

Test case added in tests/tcg/hexagon/overflow.c

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>