ENV/std: use full paths to gcc versions.
ENV/std: use full paths to gcc versions.

This is consistent with how e.g. `gcc-4.2` is handled.

This means e.g. `ghc` can find `gcc-4.8` at runtime.

Fixes Homebrew/homebrew#28934.

mahori/brew,muellermartin/dist,bfontaine/brew,reelsense/brew,gregory-nisbet/brew,DomT4/brew,alyssais/brew,alyssais/brew,zmwangx/brew,EricFromCanada/brew,bfontaine/brew,reitermarkus/brew,reitermarkus/brew,jmsundar/brew,JCount/brew,tonyg/homebrew,vitorgalvao/brew,palxex/brew,mahori/brew,muellermartin/dist,jmsundar/brew,MikeMcQuaid/brew,staticfloat/brew,zmwangx/brew,konqui/brew,pseudocody/brew,DomT4/brew,amar-laksh/brew_sudo,Linuxbrew/brew,rwhogg/brew,hanxue/linuxbrew,pseudocody/brew,gordonmcshane/homebrew,toonetown/homebrew,vitorgalvao/brew,nandub/brew,claui/brew,vitorgalvao/brew,aw1621107/brew,gordonmcshane/homebrew,mistydemeo/homebrew,Linuxbrew/brew,nandub/brew,Homebrew/brew,claui/brew,mgrimes/brew,alyssais/brew,hanxue/linuxbrew,bfontaine/brew,palxex/brew,MikeMcQuaid/brew,reitermarkus/brew,amar-laksh/brew_sudo,DomT4/brew,maxim-belkin/brew,nandub/brew,aw1621107/brew,mgrimes/brew,reelsense/brew,sjackman/homebrew,Linuxbrew/brew,rwhogg/brew,toonetown/homebrew,Linuxbrew/brew,mgrimes/brew,gregory-nisbet/brew,Homebrew/brew,mistydemeo/homebrew,EricFromCanada/brew,JCount/brew,konqui/brew,gregory-nisbet/brew,hanxue/linuxbrew,reitermarkus/brew,claui/brew,staticfloat/brew,EricFromCanada/brew,sjackman/homebrew,muellermartin/dist,vitorgalvao/brew,Homebrew/brew,ilovezfs/brew,maxim-belkin/brew,tonyg/homebrew,mahori/brew,zmwangx/brew,nandub/brew,amar-laksh/brew_sudo,sjackman/homebrew,AnastasiaSulyagina/brew,DomT4/brew,palxex/brew,staticfloat/brew,jmsundar/brew,claui/brew,konqui/brew,EricFromCanada/brew,JCount/brew,maxim-belkin/brew,pseudocody/brew,mistydemeo/homebrew,tonyg/homebrew,MikeMcQuaid/brew,rwhogg/brew,Homebrew/brew,ilovezfs/brew,mahori/brew,MikeMcQuaid/brew,AnastasiaSulyagina/brew,toonetown/homebrew,konqui/brew,aw1621107/brew,sjackman/homebrew,gordonmcshane/homebrew,reelsense/brew,AnastasiaSulyagina/brew,ilovezfs/brew,JCount/brew