Auto merge of rust-lang#105920 - MarcusCalhoun-Lopez:respect_set, r=jyn514

Respect --set=target.platform when building rustbuild itself

`--set=target.platform.cc` and `--set=target.platform.cxx` are ignored if target is quoted.

`--set=target.platform.linker` is ignored if RUSTFLAGS is not set.

Undo parts of
rust-lang@d1291dc and
rust-lang@1532fd8

Author: bors