`sed` instructions in build doesn't work as expected on Mac M1

[dynamite-bud]

sed is not the same as gnu-sed on Mac so update the build instructions to include this.
The following lines fail:

wasix-libc/build32.sh

Lines 15 to 16 in ee81c27

	sed -i 's|__wasi__|__wasix__|g' libc-bottom-half/headers/public/wasi/api_wasix.h
	sed -i 's|__wasi_api_h|__wasix_api_h|g' libc-bottom-half/headers/public/wasi/api_wasix.h