RISC-V: Fix `test' operand error with soft-float ABI being configured

Fix a:

.../sysdeps/unix/sysv/linux/riscv/configure: line 181: test: =: unary operator expected

message produced by the RISC-V configure fragment with the soft-float
ABI selected, caused by $libc_cv_riscv_float_abi evaluating to nil in
the invocation of `test $libc_cv_riscv_float_abi = no'.

	* sysdeps/unix/sysv/linux/riscv/configure.ac: Quote
	$libc_cv_riscv_float_abi in `test' invocation.
	* sysdeps/unix/sysv/linux/riscv/configure: Regenerate.