rustc_session: default to -Z plt=yes on non-x86_64
Per the discussion in #106380 plt=no isn't a great default, and rust-lang/compiler-team#581 decided that the default should be PLT=yes for everything except x86_64. Not everyone agrees about the x86_64 part of this change, but this at least is an improvement in the state of things without changing the x86_64 situation, so I've attempted making this change in the name of not letting the perfect be the enemy of the good.
Please let me know if I've messed this up somehow - I'm not wholly confident I got this right.
r? `@nikic`
c79d6be6a2