The SET_INTERRUPT_HANDLER macro in interrupt.h used an inline assembly
feature to cause GCC to generate a unique number for a trampoline
label. Clang compiled the code using that feature without generating
any compile-time errors, but it always generated the number 0,
resulting in all interrupt trampolines having the same label names.
This patch replaces the usage of that feature with local labels, which
are supported by both GCC and Clang. See
https://sourceware.org/binutils/docs/as/Symbol-Names.html for an
explanation of local labels.
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