modules: Kconfig.simplelink: Remove POSIX_API, use fine-grained opts

PR #18780 introduces a way to decouple pthread support from the general
CONFIG_POSIX_API global switch. This commit modifies the build of
SimpleLink components to take advantage of it, since SimpleLink
libraries only require pthread, sem, clock, and sleep support, not
entire POSIX API.

This fixes the build errors in the http_get sample introduced
by the merge of #18736. As such, this patch also removes
cc3220sf_launchxl exclude from sample.yaml of that sample.

Signed-off-by: Vincent Wan <vincent.wan@linaro.org>
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>