`in` operator in bjam always returns true if its first
argument has no elements[1]. This means that if `os.platform`
is empty (not detected), the construction introduced
in commit 9084161b87 sets
ABI to `aapcs` on all platforms where `os.platform` is
empty, including, e.g. loongarch64, and breaks build there.
This commit refactors the condition to use '=' operator,
to make sure that when `os.platform` is empty we get
the default ABI value, and thus fixes build on loongarch64.
See also 819c2d6423 for
similar fix that was somehow lost in refactorings. This
time a comment is added near the condition in hope
that future edits will not reintroduce the issue.
[1] https://www.boost.org/doc/libs/1_83_0/tools/build/doc/html/index.html#jam.language.flow_of_control
Fixes: 9084161b87
