tools/nolibc: mips: load current function to $t9

The MIPS calling convention requires the address of the current function
to be available in $t9.
This was not done so far.

For GCC this seems to have worked, but when compiled with clang the
executable segfault instantly.
Properly load the address of _start_c() into $t9 before calling it.

Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20240807-nolibc-llvm-v2-2-c20f2f5fc7c2@weissschuh.net


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>