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--print-file-name cannot see the library paths the specs add, so on a distro that ships picolibc outside the toolchain sysroot it answers with newlib and the image silently links a libc that is not the one being shipped.
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--print-file-nameresolves against the compiler's own search directories anddoes not consult the library paths a specs file adds. On a distro where picolibc
is the toolchain sysroot (Debian) that goes unnoticed, because the answer is
picolibc either way. On a distro that keeps picolibc outside the sysroot (Ubuntu,
/usr/lib/picolibc/<triple>/lib/) the same probe answers with newlib wheneverlibnewlib-*is also installed, and the image silently links a libc that is notthe one being shipped.
The existing tail fallback was already the correct lookup, but hardcoded to
riscv64-unknown-elfand unreachable: the probe ahead of it returns a pathrather than nothing, so
$(or ...)never advances to it.This generalises that lookup to the triple and asks it first. Debian is
unaffected (the directory does not exist, so it falls through to the specs probe
exactly as before); Ubuntu arm targets switch from newlib to picolibc, which is
what the released firmware links. Measured on openwatt's bk7231n, where newlib
costs about 20KB of flash against a 0.4% margin.