Fix MUSL ARM64 cross build

Fix MUSL ARM64 cross build

There were two issues:
* the cross/toolchain.cmake was unconditionally setting the linker to ld.gold.
  This is not wanted in general and the ld.gold cannot link arm64 MUSL stuff
  correctly (it leaves a couple of TLS symbols as undefined)
* the src/vm/arm64/asmhelpers.S was referencing a global variable using absolute
  relocation, which is not allowed in shared libraries on MUSL based Linux distros.