config: don't hardcode toolchain binaries

config: don't hardcode toolchain binaries

Currently the names of the build toolchain binaries are hardcoded in
StdGNU.mk, and the values from the environment are ignored.

Switch StdGNU.mk to use '?=' instead of '=', so that values from the
environment are used if present, else default to the values provided
by the config file.

This change fixes the gitlab CI loop, that was relying on passing
custom values in the environment variables for the compiler and the
linker.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>