devel/arm-none-eabi-newlib: * Fix excessive indentation [1]

• Be more friendly w.r.t aggressively assigning variables: Conditionally

assign NEWLIB_TARGET, and CONFIGURE_ARGS+= in case we create a slave
port from this needing a different NEWLIB_TARGET.

PR: 213565
Submitted by: Kyle Evans <bsdports@kyle-evans.net> (maintainer)
Reported by: danfe [1]