flang: fix backtrace build on FreeBSD (#120297)
FreeBSD's libexecinfo defines backtrace with a size_t for the size
argument and return type. This almost certainly doesn't make sense, but
what's done is done so cast the output to allow compilation. Otherwise
we get:
.../flang/runtime/stop.cpp:165:13: error: non-constant-expression cannot
be narrowed from type 'size_t' (aka 'unsigned long') to 'int' in
initializer list [-Wc++11-narrowing]
165 | int nptrs{backtrace(buffer, MAX_CALL_STACK)};
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
