Lua: Fix bad bitshift in lua_strx2number()

The port of lua to OpenZFS modified lua to use int64_t for numbers
instead of double. As part of this, a function for calculating
exponentiation was replaced with a bit shift. Unfortunately, it did not
handle negative values. Also, it only supported exponents numbers with
7 digits before before overflow. This supports exponents up to 15 digits
before overflow.

Clang's static analyzer reported this as "Result of operation is garbage
or undefined" because the exponent was negative.

Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14204