rustc_session: default to -Z plt=yes on non-x86_64

Per the discussion in rust-lang#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.

Author: durin42

compiler/rustc_session/src/options.rs

@@ -1613,7 +1613,7 @@ options! {
     plt: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "whether to use the PLT when calling into shared libraries;
         only has effect for PIC code on systems with ELF binaries
-        (default: PLT is disabled if full relro is enabled)"),
+        (default: PLT is disabled if full relro is enabled on x86_64)"),
     polonius: bool = (false, parse_bool, [TRACKED],
         "enable polonius-based borrow-checker (default: no)"),
     polymorphize: bool = (false, parse_bool, [TRACKED],

compiler/rustc_session/src/session.rs

@@ -1013,7 +1013,7 @@ impl Session {
     pub fn needs_plt(&self) -> bool {
         // Check if the current target usually needs PLT to be enabled.
         // The user can use the command line flag to override it.
-        let needs_plt = self.target.needs_plt;
+        let needs_plt = self.target.arch != "x86_64";
 
         let dbg_opts = &self.opts.unstable_opts;