CMake: Remove custom C library selection mechanism

Having the HAL or a HAL implementation set the C library to use results
in conflicts when linking with other libraries that did not set the same
C library. Instead of a HAL-specific way to choose the C library,
applications can provide their own custom CMake toolchain file to apply
options globally.

Signed-off-by: Jaeden Amero <jaeden.amero@arm.com>