pgsql: Tweak allocation rule for tzload()'s "union local_storage" varia

Tweak allocation rule for tzload()'s "union local_storage" variable.

By default, allocate this via malloc, as we've been doing since commit
62c8421e8. Commit aeb07c55f adopted upstream tzdb's default of
allocating it on the stack, but that still doesn't seem like a good
idea for the reasons given in 62c8421e8 (and now memorialized in a
comment, in hopes that we don't make the same mistake again).

However, under USE_VALGRIND, put it on the stack as upstream does.
This accidentally prevents a crash when Python 3.14 is used under
Valgrind. The reasons for that are obscure, and it's most likely
not our bug, and even if we figured it out it'd be nice to have a fix
for buildfarm member skink now rather than after persuading the guilty
party to fix it.

In the normal non-USE_VALGRIND case, this has no effect on the logic
in released branches, and it reverts master to match them.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/f071e691-5930-4738-9dbd-43ed2da367fe@gmail.com
Backpatch-through: 14

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/cf9453b919c75674a9349090e02dcff1c4d2a5e9

Modified Files
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src/timezone/localtime.c | 32 +++++++++++++++++++++++---------
1 file changed, 23 insertions(+), 9 deletions(-)