liblog: add __attribute__((uninitialized)) to liblog buffers

These buffers are immediately written into with *printf() and are in
the hot path, so do not initialize them.  This saves ~70ns off of each
log message, which is ~14% of the overhead within
__android_log_print() when writing to a no-op logger.

Test: liblog benchmarks
Change-Id: I2b898e72c75b57bc63fee565b49a4e00e377ed1a