backtrace: Make backtrace_capture() work on more systems.
The backtrace_capture() implementation only worked properly with GNU C on
systems that have a simple stack frame with a frame pointer. Notably,
the x86-64 ABI by default has no frame pointer, so this failed on x86-64.
However, glibc has a function named backtrace() that does what we want.
This commit tests for this function and uses it when it is present, fixing
x86-64 backtraces.