Added LCG 103

This adds the definitions needed to build against LCG 103 plus some minor improvements.

I added a number of combinations of:

• x86_64_v2, x86_64_v3, x86_64_v4+vecwid256, armv8.1_a
• centos7, el9 (currently alias to SFT centos9 builds)
• gcc11, gcc12, clang12
• opt, dbg, do0, opt+g, dbg+XYZsan

plus some +detdesc variants.

Notes

• Allen platforms not available for el9 as there are no centos9-clang12 upstream builds yet.

List of changes

• improved handling of LHCB_Core base layer
• added missing compiler definitions for el9/centos9
• added support for cuda11_8 special flag
• add definitions for LCG 103 (and layer LHCB_7)