tools/nolibc: make argc 32-bit in riscv startup code

The "ld a0, 0(sp)" instruction doesn't build on RISCV32 because that
would load a 64-bit value into a 32-bit register. But argc 32-bit,
not 64, so we ought to use "lw" here. Tested on both RISCV32 and
RISCV64.

Cc: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>