Adding support for mips and mips64

[tharunsuresh-code]

Hi everyone,

Currently, there is no support for mips and mips64 architecture. If I build libffi with the command -

cargo build --target mips64el-unknown-linux-gnuabi64

the compilation throws the following error -

error[E0425]: cannot find value `FFI_TRAMPOLINE_SIZE` in this scope
   --> libffi-sys-rs/src/lib.rs:165:25
    |
165 |     pub tramp: [c_char; FFI_TRAMPOLINE_SIZE],
    |                         ^^^^^^^^^^^^^^^^^^^ not found in this scope
    |
note: these constants exist but are inaccessible

This seems to be sorted with the following changes in arch.rs file as follows:

#[cfg(all(target_arch = "mips64", unix))]
pub use x86::x86_64::*;

#[cfg(all(target_arch = "mips", unix))]
pub use x86::x86::*;

Are these appropriate changes to make and apt for the problem mentioned?

[chenx97]

I copied constants from libffi/src/mips/ffitarget.h in #87. This should fix the issue properly.