Use libc::c_char instead of i8 due to platforms with unsigned char

src/libstd/sys/unix/backtrace/mod.rs

@@ -94,8 +94,9 @@ mod printing;
 pub mod gnu {
  use io;
  use fs;
+ use libc::c_char;
 
- pub fn get_executable_filename() -> io::Result<(Vec<i8>, fs::File)> {
+ pub fn get_executable_filename() -> io::Result<(Vec<c_char>, fs::File)> {
  Err(io::Error::new(io::ErrorKind::Other, "Not implemented"))
  }
 }

src/libstd/sys/windows/backtrace_gnu.rs

@@ -10,6 +10,7 @@
 
 use io;
 use sys::c;
+use libc::c_char;
 use path::PathBuf;
 use fs::{OpenOptions, File};
 use sys::ext::fs::OpenOptionsExt;
@@ -53,7 +54,7 @@ fn lock_and_get_executable_filename() -> io::Result<(PathBuf, File)> {
 // Get the executable filename for libbacktrace
 // This returns the path in the ANSI code page and a File which should remain open
 // for as long as the path should remain valid
-pub fn get_executable_filename() -> io::Result<(Vec<i8>, File)> {
+pub fn get_executable_filename() -> io::Result<(Vec<c_char>, File)> {
  let (executable, file) = lock_and_get_executable_filename()?;
  let u16_executable = to_u16s(executable.into_os_string())?;
  Ok((wide_char_to_multi_byte(c::CP_ACP, c::WC_NO_BEST_FIT_CHARS,