[compiler-rt][GWP-ASan] Use no-op backtrace if execinfo.h missing (#201266) Guard backtrace_linux_libc.cpp with __has_include(<execinfo.h>) so that targets without it (e.g. musl libc) get null stubs instead of a build failure. GitOrigin-RevId: 252ad2052409a33eff59a149053ec90f7fae03df