RISC-V: Fixbug for fsflags instruction error using immediate.

The pattern mistakenly believes that fsflags can use immediate numbers,
but in fact it does not support it. Immediate numbers should use fsflagsi.

For example:
__builtin_riscv_fsflags(4);

The following error occurred.
/tmp/ccoWdWqT.s: Assembler messages:
/tmp/ccoWdWqT.s:14: Error: illegal operands `fsflags 4'

gcc/ChangeLog:

	* config/riscv/riscv.md: Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/fsflags.c: New test.