tests/subsys/logging/log_backend_uart: Reduce printf size

When using picolibc before 1.8.5, the only way to get 'long long' support
was to use the full version, including floating point support. This is too
large for this testcase.

Reduce the size of the printf code by switching to the version without
64-bit integer support. This allows the test to pass when using older
picolibc versions, such as that included with SDK version 0.16.3.

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