[feature request] Support linux-ppc64le

[runlevel5]

When I tried to compile the lib, I got following errors:

$ cargo build
    Updating crates.io index
     Locking 24 packages to latest compatible versions
   Compiling rustversion v1.0.17
   Compiling libc v0.2.155
   Compiling cfg-if v1.0.0
   Compiling log v0.4.22
   Compiling generator v0.8.2 (/home/tle/Work/generator-rs)
error[E0583]: file not found for module `asm`
  --> src/detail/mod.rs:31:1
   |
31 | pub mod asm;
   | ^^^^^^^^^^^^
   |
   = help: to create the module `asm`, create file "src/detail/asm.rs" or "src/detail/asm/mod.rs"
   = note: if there is a `mod asm` elsewhere in the crate already, import it with `use crate::...` instead

error[E0432]: unresolved imports `self::asm::gen_init`, `self::asm::initialize_call_frame`, `self::asm::swap_registers`, `self::asm::InitFn`, `self::asm::Registers`
  --> src/detail/mod.rs:35:21
   |
35 | pub use self::asm::{gen_init, initialize_call_frame, swap_registers, InitFn, Registers};
   |                     ^^^^^^^^  ^^^^^^^^^^^^^^^^^^^^^  ^^^^^^^^^^^^^^  ^^^^^^  ^^^^^^^^^ no `Registers` in `detail::asm`
   |                     |         |                      |               |
   |                     |         |                      |               no `InitFn` in `detail::asm`
   |                     |         |                      no `swap_registers` in `detail::asm`
   |                     |         no `initialize_call_frame` in `detail::asm`
   |                     no `gen_init` in `detail::asm`

error[E0412]: cannot find type `ucontext_t` in crate `libc`
  --> src/stack/overflow_unix.rs:29:21
   |
29 |     ctx: *mut libc::ucontext_t,
   |                     ^^^^^^^^^^ not found in `libc`

Some errors have detailed explanations: E0412, E0432, E0583.
For more information about an error, try `rustc --explain E0412`.
error: could not compile `generator` (lib) due to 3 previous errors
Exit 101

If you need SSH access to an ppc64le VM for troubleshooting, please feel free to let me know