build: use new toolchain variables for tools

This change migrates the values of `CC`, `CPP`, `AS` and other toolchain
variables to the new `$(toolchain)-$(tool)` variables, which were
introduced by the toolchain refactor patch. These variables should be
equivalent to the values that they're replacing.

Change-Id: I644fe4ce82ef1894bed129ddb4b6ab94fb04985d
Signed-off-by: Chris Kay <chris.kay@arm.com>
37 files changed