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 {