Add XNN_HAS_MMAP to common.h, as a way to indicate that a given platf… #1239

Job	Run time
build-and-test / cmake-linux-riscv64	8m 4s
build-and-test / cmake-linux-armhf	11m 57s
build-and-test / cmake-linux-aarch64	10m 57s
build-and-test / cmake-linux-local	12m 52s
build-and-test / cmake-linux-riscv64-rvv	11m 15s
build-and-test / cmake-windows-arm64	8m 23s
build-and-test / cmake-windows-x64	10m 28s
build-and-test / bazel-linux-aarch64	14m 51s
build-and-test / bazel-linux-x86_64	14m 9s
build-and-test / bazel-linux-x86_64-gcc-9	8m 38s
build-and-test / cmake-android (build-android-arm64.sh)	10m 55s
build-and-test / cmake-ios-arm64	5m 22s
build-and-test / cmake-android (build-android-armv7.sh)	10m 13s
build-and-test / cmake-ios-x86_64	7m 6s
build-and-test / cmake-macos-arm64	9m 23s
build-and-test / cmake-android (build-android-x86.sh)	12m 27s
build-and-test / cmake-windows-x86	11m 36s
	2h 58m 36s

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