Improvements for aarch64 support

Makefile

src/ld_so/ld_script/aarch64-unknown-redox.ld

src/ld_so/ld_script/x86_64-unknown-linux.ld

src/ld_so/ld_script/x86_64-unknown-redox.ld

src/ld_so/src/lib.rs

src/ld_so/tcb.rs

src/platform/redox/mod.rs

src/platform/redox/ptrace.rs