Default to Pentium 4 as the x86 target CPU on Windows/Linux/DragonFly

Limiting ourselves to a generic x86 instruction set doesn't seem useful.
Both users running those systems on original i386 hardware might as well
manually specify a target cpu ;-)

Clang uses the same default.

src/librustc_back/target/i686_pc_windows_gnu.rs

@@ -12,6 +12,7 @@ use target::Target;
 
 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 @@ use target::Target;
 
 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 @@ use target::Target;
 
 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 {