bootstrap: Avoid redundant compiler selection checks
In !1535 (merged) an extra level of nesting was
added to the selection loop, but the inner-most break
command used to
exit the loop on success was not updated. This caused the outer-most
loop to iterate unnecessarily and repeatedly try the same compilers
again. In the case of compilers requiring a -std=
flag, this may have
caused the oldest standard to be used instead of the newest.