The js/wasm architecture does not support signals at all, but there are
already some signal constants defined because of stdlib dependencies.
This change adds a dummy constant for syscall.SIGTERM as well, to make
js/wasm compatible with more existing Go code.
There is the Go proverb "Syscall must always be guarded with build
tags.", so code should not expect syscall.SIGTERM to exist. Still,
adding SIGTERM should do more good than harm.