gcc/picolibc: Disable -fprintf-return-value when printf is not complete

GCC will compute expected sprintf (et al) return values internally and use
them in place of the actual return value. When the printf implementation
has reduced functionality, gcc may compute a different value.

For picolibc, this means disabling the optimization unless floating point
output is enabled.

Signed-off-by: Keith Packard <keithp@keithp.com>