docs: Bump Arm Compiler version

Bump the version of the Arm Compiler to 6.18 as mandated by TF-M for
patching the VLLDM Instruction Security Vulnerability[1]. Replace dead
links with working ones. Change the name to "Arm Compiler for Embedded"
to distinguish it from "Arm Compiler for Linux".

[1] https://developer.arm.com/Arm%20Security%20Center/VLLDM%20Instruction%20Security%20Vulnerability



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