Build libbacktrace with mmapio.c instead of read.c

Fixes #289

`mmap` should be available on all platforms we support, so we can use
`libbacktrace`'s more efficient mmap-based code.

Author: Aaron1011

crates/backtrace-sys/build.rs

@@ -31,7 +31,7 @@ fn main() {
         .file("src/libbacktrace/dwarf.c")
         .file("src/libbacktrace/fileline.c")
         .file("src/libbacktrace/posix.c")
-        .file("src/libbacktrace/read.c")
+        .file("src/libbacktrace/mmapio.c")
         .file("src/libbacktrace/sort.c")
         .file("src/libbacktrace/state.c");