[Bug target/89955] riscv.h improperly defines STARTFILE_PREFIX_SPEC spec

[kallisti5 at unixzen dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89955

--- Comment #8 from Alexander von Gluck <kallisti5 at unixzen dot com> ---
Looks good to me.  Now also matches other architectures.
Thanks! (viva riscv)