pgsql: Fix platform-specific performance regression in logtape.c.

Fix platform-specific performance regression in logtape.c.

Commit 24d85952 made a change that indirectly caused a performance
regression by triggering a change in the way GCC optimizes memcpy() on
some platforms.

The behavior seemed to contradict a GCC document, so I filed a report:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95556

This patch implements a narrow workaround which eliminates the
regression I observed. The workaround is benign enough that it seems
unlikely to cause a different regression on another platform.

Discussion: https://postgr.es/m/99b2eab335c1592c925d8143979c8e9e81e1575f.camel@j-davis.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1fbb6c93df30801f83c6804ab7befde3cdefe677

Modified Files
--------------
src/backend/utils/sort/logtape.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)