fix(Makefile): delay

I noticed a pretty significant delay (of ~5s on my machine) every time I executed make in relibc.

This was because TARGET is currently defined as a recursively expanded variable. Which in turn means that the command:

rustc -Z unstable-options --print target-spec-json | grep llvm-target | cut -d '"' -f4

will get run everytime TARGET is mentioned. This patch fixes the above by defining the variable as an immediately expanded value. This will ensure the command only get run once and still skips if predefined.

Signed-off-by: Anhad Singh andypython@protonmail.com