Silenced conversion warnings in src/core/support

src/core/support/cmdline.c

src/core/support/cpu_linux.c

src/core/support/file.c

src/core/support/histogram.c

src/core/support/host_port.c

src/core/support/murmur_hash.c

src/core/support/slice.c

src/core/support/slice_buffer.c

src/core/support/string.c

src/core/support/subprocess_posix.c

src/core/support/time.c

src/core/support/time_posix.c