gcc: Fix registers saved in software floating point mode.

The floating point registers should only be saved if the relevant hardware is
present.
gcc: Add soft-float support to libgcc.

Add software floating point functions to libgcc using fp-bit.c.  Format
conversion stubs (riscv-fp.c) are based on rs6000/ppc64-fp.c with TF types
removed.
glibc: Fix typo in preprocessor conditional.

The appropriate symbol is __riscv_soft_float, not __mips_soft_float.
Merge pull request #4 from darius-bluespec/softfloat

Add software floating point support.