pgsql: Convert archiver's force_dir_scan variable to an atomic variable

Convert archiver's force_dir_scan variable to an atomic variable.

Commit bd5132db55 introduced new atomic read/write functions with
full barrier semantics, which are intended to simplify converting
non-performance-critical code to use atomic variables.  This commit
demonstrates one such conversion.

Reviewed-by: Yong Li
Discussion: https://postgr.es/m/20231110205128.GB1315705%40nathanxps13

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3179701505b9ec58243ffba9d22b1803f320bf60

Modified Files
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src/backend/postmaster/pgarch.c | 21 +++++----------------
1 file changed, 5 insertions(+), 16 deletions(-)