[3.14] Being more flexible in when not to explicitly set the sysroot when compiling for WASI (GH-142242) (GH-142246)

Being more flexible in when not to explicitly set the sysroot when compiling for WASI (GH-142242)
(cherry picked from commit c5252045d3a7164f1829503d122091b5e469fda3)

Co-authored-by: Brett Cannon <brett@python.org>