Fix ABI detection for empty 'os.platform'

[iv-m]

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 9084161 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 819c2d6 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: 9084161

[iv-m]

I faced this issue when building boost 1.87.0 on loongarch64. The b2 output and fix are similar to #234:

error: No best alternative for /usr/src/RPM/BUILD/boost-1.87.0/libs/context/build/asm_sources with <abi>aapcs <address-model>64 <architecture>loongarch <asynch-exceptions>off <binary-format>elf <boost.beast.allow-deprecated>on <boost.beast.separate-compilation>on <boost.cobalt.executor>any_io_executor <boost.cobalt.pmr>std <boost.process.fs>boost <context-impl>fcontext <coverage>off <debug-symbols>off <exception-handling>on <extern-c-nothrow>off <inlining>full <known-warnings>hide <link>shared <optimization>off <os>LINUX <pch>off <preserve-test-targets>on <profiling>off <python-debugging>off <python>3.12 <relevant>abi <relevant>address-model <relevant>architecture <relevant>binary-format <relevant>toolset <rtti>on <runtime-debugging>off <runtime-link>shared <stdlib>native <strip>off <target-os>linux <testing.execute>on <threadapi>pthread <threading>multi <toolset-gcc:version>14 <toolset>gcc <variant>release <vectorize>off <visibility>hidden <warnings-as-errors>off <warnings>on <x-deduced-platform>loongarch_64

[olk]

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