Improve performance of pg_strtointNN functions

Experiments have shown that modern versions of both gcc and clang are
unable to fully optimize the multiplication by 10 that we're doing in the
pg_strtointNN functions.  Both compilers seem to be making use of "imul",
which is not the most efficient way to multiply by 10.  This seems to be
due to the overflow checking that we're doing.  Without the overflow
checks, both those compilers switch to a more efficient method of
multiplying by 10.  In absence of overflow concern, integer multiplication
by 10 can be done by bit-shifting left 3 places to multiply by 8 and then
adding the original value twice.

To allow compilers this flexibility, here we adjust the code so that we
accumulate the number as an unsigned version of the type and remove the
use of pg_mul_sNN_overflow() and pg_sub_sNN_overflow().  The overflow
checking can be done simply by checking if the accumulated value has gone
beyond a 10th of the maximum *signed* value for the given type.  If it has
then the accumulation of the next digit will cause an overflow.  After
this is done, we do a final overflow check before converting the unsigned
version of the number back to its signed counterpart.

Testing has shown about an 8% speedup of a COPY into a table containing 2
INT columns.

Author: David Rowley, Dean Rasheed
Discussion: https://postgr.es/m/CAApHDvrL6_+wKgPqRHr7gH_6xy3hXM6a3QCsZ5ForurjDFfenA@mail.gmail.com
Discussion: https://postgr.es/m/CAApHDvrdYByjfj-=WbmVNFgmVZg88-dE7heukw8p55aJ+W=qxQ@mail.gmail.com

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