time64: Remove leading whitespace from GLIBC_64BIT_TIME_FLAGS

This avoids adding a spurious space in TARGET_CC_ARCH when
GLIBC_64BIT_TIME_FLAGS is empty

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>