Enable `std` for rust-psp

[MrAwesome]

Highlights:

• Working std out of the box! Downstream crates only need to enable the restricted_std feature in place of adding no_std.
• Simpler cargo psp code by moving to cargo -Z build-std.
• "std" feature that can be turned off to speed up no_std crate builds (just requires adding no_std in main.rs and default-features = false in Cargo.toml) [NOTE: this is turned off by default for now, since std is mostly unusable at the moment]
• Conditional compilation of imports and functions that clash with std

Remaining questions:

• Does the conditional compilation look right? I mostly just Made The Compiler Happy here so there may be better places to do the std/no_std splits.
• Should rust_eh_personality have a particular implementation? Most implementations of this are empty, only Windows does anything with it afaik.

Test Plan:

[Mon 20/07/27 22:23 PDT][pts/0]                                                                                                         
<glenn@ronnie:~/rust-psp/examples/hello-world>                                                                                          
$ cargo psp                                                                                                                             
    Finished dev [unoptimized + debuginfo] target(s) in 0.04s                                                                           
Saved to /home/glenn/rust-psp/examples/hello-world/target/mipsel-sony-psp/debug/EBOOT.PBP                                               
[Mon 20/07/27 22:23 PDT][pts/0]                                                                                                         
<glenn@ronnie:~/rust-psp/examples/hello-world>                                                                                          
$ cd ../rust-std-hello-world                                                                                                            
[Mon 20/07/27 22:23 PDT][pts/0]                                                                                                         
<glenn@ronnie:~/rust-psp/examples/rust-std-hello-world>                                                                                 
$ cargo psp                                                                                                                             
    Finished dev [unoptimized + debuginfo] target(s) in 0.04s                                                                           
Saved to /home/glenn/rust-psp/examples/rust-std-hello-world/target/mipsel-sony-psp/debug/EBOOT.PBP   

(Note that this was with a locally patched libc via symlinks (yuck), as 0.2.74 has not launched yet.)

[sajattack]

Niiiiiiiiiice.