Add macos 15 to CI

[rikkimax]

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Testing this PR locally

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dub run digger -- build "master + dmd#22098"

[rikkimax]

I've added a bunch of different configurations for the bootstrap compiler and which image for 15.

macOS 15 intel x64, DMD (bootstrap) can basically be ignored, it segfaults when trying to build build.d.

[rikkimax]

Main / macOS 15 x64, LDC (bootstrap) (pull_request) exemplifies: https://github.com/dlang/dmd/issues/21987

[rikkimax]

Main / macOS 15 intel x64, LDC (bootstrap) (pull_request) is the interesting one.

+ld: warning: ignoring extranenous label 'L_OBJC_LABEL_CLASS_$' at end of section '__objc_classlist'

Test 'runnable/test15779.d' failed: caught signal: -11

A huge number of (different object files and symbols):

could not find referenced DIE
note: while processing /Users/runner/work/dmd/dmd/generated/osx/debug/64/libdruntime.a(monitor__c13_428.o)

[rikkimax]

@WalterBright here ya go.