platforms: Common Entry Point for GCC/Clang

In the CMakelists file of each platform, set Reset_Handler as the
entry point. This ensures that the same entry point is used for both
Clang and GCC.

Therefore, to keep things homogeneous, remove the gcc entry points specified
in the linker files.

Signed-off-by: Bilal Wasim <muhammadbilal.wasim@arm.com>