Use syscall from libc on Linux instead of redefining it

[samueltardieu]

libc is already required. Fix #610.

[dhardy]

Build failed on Solaris:

$ cargo build --target=x86_64-sun-solaris --all-features
    Updating crates.io index
 Downloading crates ...
  Downloaded syn v0.15.9
  Downloaded proc-macro2 v0.4.20
   Compiling proc-macro2 v0.4.20
   Compiling unicode-xid v0.1.0
   Compiling semver-parser v0.7.0
   Compiling serde v1.0.79
   Compiling cfg-if v0.1.5
   Compiling libc v0.2.43
   Compiling semver v0.9.0
   Compiling packed_simd v0.3.0
   Compiling log v0.4.5
   Compiling rustc_version v0.2.3
   Compiling quote v0.6.8
   Compiling rand_chacha v0.1.0 (/home/travis/build/rust-random/rand/rand_chacha)
   Compiling rand v0.5.5 (/home/travis/build/rust-random/rand)
   Compiling syn v0.15.9
   Compiling serde_derive v1.0.79
   Compiling rand_core v0.3.0 (/home/travis/build/rust-random/rand/rand_core)
   Compiling rand_xorshift v0.1.0 (/home/travis/build/rust-random/rand/rand_xorshift)
   Compiling rand_hc128 v0.1.0 (/home/travis/build/rust-random/rand/rand_hc128)
   Compiling rand_isaac v0.1.0 (/home/travis/build/rust-random/rand/rand_isaac)
error[E0425]: cannot find function `syscall` in module `libc`
   --> src/rngs/os.rs:687:19
    |
687 |             libc::syscall(SYS_GETRANDOM, buf.as_mut_ptr(), buf.len(),
    |                   ^^^^^^^ not found in `libc`

[samueltardieu]

@dhardy This can only be used on Linux indeed. I pushed an updated patch which lets the Solaris part untouched.

[dhardy]

Thanks @samueltardieu!