gcc: use '.br_real' instead of '.real' suffix for the raw internal toolchain

If an externally built (non-Buildroot) toolchain also wraps the toolchain
executables, there is a risk that it will also use the '.real' extension.
To minimise this risk, use a more buildroot-specific extension instead:
'.br_real', so we can detect that the external toolchain is built using
Buildroot and get to the raw toolchain binaries.

[Peter: reword description]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>