build: Don't set `-fno-threadsafe-statics` on macOS

This flag is not set on other platforms so it can produce inconsistent
behaviour across platforms. For example, if you build an async node
add-on which uses statics you can get race conditions due to static
not supporting threads if the node add-on inherits from the Node
common.gypi config. It is not disabled on other platforms such as
Linux, it is not disabled by default in Xcode or clang.

This setting has been there since the initial commit that introduces
`common.gypi` and thus has been there since the start, it doesn't seem
to be have added for any particular reason other than to potentially
match the Xcode defaults at the time.

PR-URL: https://github.com/nodejs/node/pull/22198


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>