Revert "Prefer bundled gcc.  External gcc can still be used if one provides a full path via -Clinker=..."

This reverts commit 94f05324fee6cd4637adc01f441fafd09e678668.
auto merge of #17443 : alexcrichton/rust/issue-17442, r=brson

...des a full path via -Clinker=..."

This reverts commit 94f05324fee6cd4637adc01f441fafd09e678668.