Compile libbacktrace with mmapio.c instead of read.c

[Aaron1011]

Currently, backtrace-sys hardcoded the compilation of src/libbacktrace/read.c:

backtrace-rs/crates/backtrace-sys/build.rs

Line 33 in 190b2f9

.file("src/libbacktrace/read.c")

However, libbacktrace provides a more efficient version of read.c (which is used internally to read in chunks of memory) called mmapio.c.

backtrace-rs should either provide a feature which compiles libbacktrace with mmapio.c, or attempt to detect whether or not the mmap system call is available. Alternatively, the configure script for libbacktrace could be used instead of hard-coding paths, since [it already detects if mmap is available[(https://github.com/ianlancetaylor/libbacktrace/blob/559ab7cab4a6002124863d493bd09a376a690e76/configure.ac#L285-L303). I'm not sure if there are any issues preventing this script from being used.

This will also fix rust-lang/rust#69151, since we will no longe be hitting the bug in read.c

[alexcrichton]

A PR to implement this would be most welcome! I don't think anything fancy needs to be done, mmap is available everywhere reasonable.