fcntl adding F_RDAHEAD for apple targets. #1381

Job	Run time
minver	16s
linux_native_builds (x86_64-unknown-linux-gnu)	4m 30s
macos	9m 52s
macos-aarch64	3m 32s
linux_native_builds (x86_64-unknown-linux-musl)	4m 12s
rust_stable	1m 35s
rustfmt	15s
tier3 (x86_64-unknown-dragonfly)	59s
tier3 (x86_64-unknown-openbsd)	1m 0s
tier3 (x86_64-unknown-haiku)	1m 3s
redox	43s
cross (arm-unknown-linux-gnueabi)	1m 55s
cross_compiling (aarch64-linux-android)	49s
cross (armv7-unknown-linux-gnueabihf)	1m 50s
cross_compiling (arm-linux-androideabi)	38s
cross (i686-unknown-linux-gnu)	1m 57s
cross_compiling (armv7-linux-androideabi)	38s
cross (i686-unknown-linux-musl)	1m 59s
cross_compiling (i686-linux-android)	37s
cross (mips-unknown-linux-gnu)	1m 45s
cross_compiling (x86_64-linux-android)	35s
cross (mips64-unknown-linux-gnuabi64)	2m 1s
cross_compiling (arm-unknown-linux-musleabi)	40s
cross_compiling (x86_64-unknown-fuchsia)	34s
cross_compiling (x86_64-unknown-illumos)	30s
cross (mips64el-unknown-linux-gnuabi64)	1m 58s
cross (mipsel-unknown-linux-gnu)	1m 43s
cross_compiling (aarch64-apple-ios, true)	41s
cross_compiling (powerpc64-unknown-linux-gnu)	40s
cross (powerpc64le-unknown-linux-gnu)	1m 46s
cross_compiling (s390x-unknown-linux-gnu)	39s
cross_compiling (x86_64-unknown-linux-gnux32)	44s
cross_compiling (x86_64-unknown-netbsd)	35s
	53m 11s

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