signbit macro uses __signbitf function although not compiled in?

[andreasf]

The public header posix/math.h defines a signbit macro to call functions __signbitf et al. However these cannot be resolved at runtime.

The implementation for this function seems to be in libroot/posix/glibc/arch/generic/s_signbitf.c but that directory does not seem to be compiled in - there's no ./Jamfile, and ../Jamfile only includes the specific architecture.

Mono's configure for instance detects signbit but later fails to link. (It can work without, but gets confused by the current situation.)

[korli]

Added s_signbitf.c for x86 in r27259