Auto merge of #28331 - semarie:openbsd-configure-egcc, r=alexcrichton

Auto merge of #28331 - semarie:openbsd-configure-egcc, r=alexcrichton

under OpenBSD, it could be have present multiples versions of gcc compiler:
  - gcc-4.2 (with patchs) : c/c++ compiler present in `/usr/bin`. It is unusable to build recent LLVM (so rustc too).
  - gcc/g++ -4.9 : c/c++ compiler, installed as third-party with "ports" mechanism. The compiler is installed in `/usr/local` as `egcc` (and `eg++`).

this PR adds probing for `egcc` for `CFG_GCC`, if the first probed `gcc` is too old. It will set `CC` and `CXX` too, in order to pass the variables to LLVM configure if builded.

please note this PR is a first step in order to build rustc under OpenBSD with unpatched tree.

r? @alexcrichton

cc @brson @dhuseby