The EXECUTABLE build property holds a CMake target name, not a filesystem
path. Under Build system v1 the target happens to be created as
"<project>.elf", so concatenating BUILD_DIR with EXECUTABLE produced a valid
elf path by coincidence. Under Build system v2 the target is just "<project>"
and the elf is written as "<project>.elf" via CMake's executable suffix, so
the same concatenation pointed gdb at a non-existent file.
Construct the path explicitly from EXECUTABLE_NAME plus the .elf suffix so
the gdbinit consumer no longer depends on the producer naming its target
after the output file.
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