[PATCH] Use ssup_datum_*_cmp for int2, oid, and oid8 sort support

Hi,

While auditing nbtcompare.c, I noticed that int2, oid, and oid8
sortsupport functions use custom local fastcmp helpers that are
functionally equivalent to the existing ssup_datum_int32_cmp (for
int2) and ssup_datum_unsigned_cmp (for oid, oid8).

This prevents these types from hitting the radix sort fast path
added in commit ef3c3cf6d02 [1], which dispatches based on the
comparator function pointer.

The original 2021-2022 thread that introduced the ssup_datum_*_cmp
helpers (commit 6974924347c, Apr 2022) [2] covered int4, int8,
timestamp, date, and the abbreviated-key types (text, uuid, macaddr,
inet, bytea).  int2 and oid weren't called out in that discussion,
and oid8 didn't exist at the time, it was added in Jan 2026 [3]
and inherited the custom fastcmp pattern from oid, about a month
before radix sort landed.  This patch fills those gaps.

Switching to the existing helpers makes these types eligible for
radix sort.  Benchmark (10M-row single-key sort):

  Type     Before    After     Speedup
  ----     ------    -----     -------
  int2     2440 ms   1778 ms   ~27%
  oid      2875 ms   2073 ms   ~28%
  oid8     2837 ms   2042 ms   ~28%
  int4     --        1765 ms   (baseline)
  int8     --        2031 ms   (baseline)

The patch just replaces the comparator assignment and removes the
now-unused local fastcmp functions.  No behavioral change and the
helpers produce identical results.

int2 uses ssup_datum_int32_cmp because there is no int16-specific
helper, every int16 fits losslessly in int32, and int32_cmp is more
efficient than signed_cmp (4-byte radix passes instead of 8).

Other custom fastcmp users in core (float4/float8, varlena types)
cannot be trivially switched due to NaN handling or locale-dependent
comparison, so they are left as-is.

Tested with make check (245/245 pass) and make isolation/check
(128/128 pass).

[1]
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=ef3c3cf6d02
[2]
https://www.postgresql.org/message-id/flat/CA%2BhUKGJ2-eaDqAum5bxhpMNhvuJmRDZxB_Tow0n-gse%2BHG0Yig%40mail.gmail.com
[3] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=b139bd3b6

Thanks,
Baji Shaik