[clang][test][RISCV] Fix precision of CHECK-NOT lines in riscv-target-features.c

`CHECK-NOT: __riscv_f 2000000{{$}}` was overly precise, in that it
wouldn't test what we want it to if the version of the F extension was
bumped. Meanwhile, lines like `CHECK-NOT: __riscv_d` were too imprecise,
as they would match any define with that prefix (e.g.
`__riscv_dsomethingelse`).

This patch just updates these lines to check for `__riscv_foo {{.*$}}`.