ccgo v4.36.0 linked musl's __builtin_nanf("") to nanf itself, so in
v1.77.0 Xnanf calls itself on every musl target. nan(), nanf(), nanl()
and strtod(), strtof(), strtold() or sscanf() of "nan" die with a fatal
stack overflow. ccgo fixes it in "v4/lib: link a builtin to the plain
function only when linking with libc".

TestNaN checks those, and "inf", in a child process, so the fatal error
fails only this test. It fails with the ccgo_linux_*.go files of v1.77.0
and passes once they are regenerated by the fixed ccgo. The hand-written
ports do not have the bug and most of them lack nan and nanl, so the
test is musl only.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Its Xnanf calls itself on the musl targets. See the parent commit.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>