riscv: Add vector registers to __SYSCALL_CLOBBERS

riscv: Add vector registers to __SYSCALL_CLOBBERS

The Linux kernel ABI specifies that the vector registers are not preserved
across system calls, but the __SYSCALL_CLOBBERS macro doesn't mention them.
This could possibly lead to compilers trying to keep data in the vector
registers across the syscall leading to corruption.  Add the vector registers
to __SYSCALL_CLOBBERS when the vector extension is enabled.  If the vector
extension is enabled, then require GCC 15 or later and RVV 1.0 or later.

Fixes: 36960f0c76 ("RISC-V: Linux Syscall Interface")
Signed-off-by: Peter Bergner <bergner@tenstorrent.com>