gdb/riscv: Update test to handle targets without an fpu

gdb/riscv: Update test to handle targets without an fpu

The FPU is optional on RISC-V.  The gdb.base/float.exp test currently
assumes that an fpu is always available on RISC-V.  Update the test so
that this is not the case.

gdb/testsuite/ChangeLog:

	* gdb.base/float.exp: Handle RISC-V targets without an FPU.