Auto merge of #21646 - dotdash:default_target_cpu, r=Aatch

Using `generic` as the target cpu limits the generated code to the bare basics for the arch, while we can probably assume that we'll actually be running on somewhat modern hardware. This updates the default target CPUs for the x86 and x86_64 archs to match clang's behaviour.

Refs #20777

src/librustc_back/target/i686_apple_darwin.rs

@@ -12,6 +12,7 @@
 
 pub fn target() -> Target {
     let mut base = super::apple_base::opts();
+    base.cpu = "yonah".to_string();
     base.pre_link_args.push("-m32".to_string());
 
     Target {

src/librustc_back/target/i686_pc_windows_gnu.rs

@@ -12,6 +12,7 @@
 
 pub fn target() -> Target {
     let mut options = super::windows_base::opts();
+    options.cpu = "pentium4".to_string();
 
     // Mark all dynamic libraries and executables as compatible with the larger 4GiB address
     // space available to x86 Windows binaries on x86_64.

src/librustc_back/target/i686_unknown_dragonfly.rs

@@ -12,6 +12,7 @@
 
 pub fn target() -> Target {
     let mut base = super::dragonfly_base::opts();
+    base.cpu = "pentium4".to_string();
     base.pre_link_args.push("-m32".to_string());
 
     Target {

src/librustc_back/target/i686_unknown_linux_gnu.rs

@@ -12,6 +12,7 @@
 
 pub fn target() -> Target {
     let mut base = super::linux_base::opts();
+    base.cpu = "pentium4".to_string();
     base.pre_link_args.push("-m32".to_string());
 
     Target {

src/librustc_back/target/x86_64_apple_darwin.rs

@@ -12,7 +12,7 @@
 
 pub fn target() -> Target {
     let mut base = super::apple_base::opts();
-    base.cpu = "x86-64".to_string();
+    base.cpu = "core2".to_string();
     base.eliminate_frame_pointer = false;
     base.pre_link_args.push("-m64".to_string());