Change the architecture constant RISCV64

[julianmorillo]

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This constant value is returned by get_cpu_architecture, which is called from get_arch_prefix, that returns it directly (lower case) to _get_gcc_prefix, causing the following error when trying to build LLVM in RISCV:

== FAILED: Installation ended unsuccessfully: Can't find GCC version 13.2.0 for architecture risc-v-64 in
/cvmfs/riscv.eessi.io/versions/20240402/software/linux/riscv64/generic/software/GCCcore/13.2.0/lib/gcc/risc-v-64-*/13.2.0 (took 1 min 25 secs)
== Results of the build can be found in the log file(s) /tmp/eb-0kc4ln6r/easybuild-LLVM-20.1.5-20250605.161517.VhpnA.log

ERROR: Installation of LLVM-20.1.5-GCCcore-13.2.0.eb failed: "Can't find GCC version 13.2.0 for architecture risc-v-64 in /cvmfs/riscv.eessi.io/versions/20240402/software/linux/riscv64/generic/software/GCCcore/13.2.0/lib/gcc/risc-v-64-*/13.2.0"

The correct path in my case would be /cvmfs/riscv.eessi.io/versions/20240402/software/linux/riscv64/generic/software/GCCcore/13.2.0/lib/gcc/riscv64-*/13.2.0