swicth GCC-system and GCCcore-system to SystemCompilerGCC easyblock

[lexming]

Companion PR to

• split SystemCompilerGCC out of SystemCompiler and adopt module_load_environment easybuild-easyblocks#3559

[lexming]

Test report by @lexming
SUCCESS
Build succeeded for 2 out of 2 (2 easyconfigs in total)
login2.cerberus.os - Linux Rocky Linux 8.10 (Green Obsidian), x86_64, Intel(R) Xeon(R) Gold 6126 CPU @ 2.60GHz, Python 3.9.20
See https://gist.github.com/lexming/02bfd3fdaaae9faa8a156bca0f789a15 for a full test report.

[branfosj]

Test report by @branfosj
SUCCESS
Build succeeded for 2 out of 2 (2 easyconfigs in total)
owl - Linux Ubuntu 24.04.2 LTS (Noble Numbat), x86_64, 13th Gen Intel(R) Core(TM) i5-1335U (skylake), Python 3.12.3
See https://gist.github.com/branfosj/4935e6064728e029e1bef65c06013da7 for a full test report.

[branfosj]

Going in, thanks @lexming!