[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
1 file changed