docs: Improve wording of toolchain support

Remove the notion that toolchain is the same as compiler which is
inaccurate, because other tools like linker, standard libraries and
more are also included in a toolchain.

Remove mentioning of the Mbed OS style of toolchain names, i.e. ARM
and GCC_ARM, as we no longer pass those values for toolchain selection
following the switch to toolchain files. Replace them with the C
compiler names, armclang and arm-none-eabi-gcc, which are useful for
querying toolchain versions.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>