libc/softfloat: we don't export _fp[gs]et* symbols

Remove attempts to use NetBSD weak_alias macros to export _-prefixed
versions of various fp[sg]et* symbols under softfloat. weak_alias
isn't defined so we didn't export them and thus the Symbol.map entries
were wrong.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D44327