PMconfig.R::getBits() issue for macOS 15.5

[mhovd]

Discussed in #264

^{Originally posted by graemeok June 19, 2025}
On macOS 15.5, PMrun() kept recompiling binaries because pNPeng.o was not present.
Tracked down to PMconfig.R::getBits() returning 32
if (length(grep("64-bit", utils::sessionInfo())) > 0) { return(64) } else { return(32) } gives 32
because "64-bit" is not present in utils::sessionInfo()[2] = "aarch64-apple-darwin20"