Rollup merge of #47110 - EdSchouten:cloudabi-tls, r=kennytm

Use the right TLS model for CloudABI.

CloudABI doesn't do dynamic linking. For this reason, there is no need
to handle any other TLS model than local-exec. CloudABI's C library
doesn't provide a __tls_get_addr() function to do Dynamic TLS.

By forcing local-exec to be used here, we ensure that we don't generate
function calls to __tls_get_addr().