Update gcc leg from v14 to v15

[am11]

https://gcc.gnu.org/gcc-15/

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[am11]

v15 is picked up https://dev.azure.com/dnceng-public/public/_build/results?buildId=1028849&view=logs&j=9d44ffa5-6dbb-5c44-43bb-984541de6129&t=2c1dca5f-db26-547e-262f-8b403ec0d74c

  Commencing build of "install" target in "CoreCLR component" for linux.x64.Checked in /__w/1/s/artifacts/obj/coreclr/linux.x64.Checked
  Invoking "/__w/1/s/eng/native/gen-buildsys.sh" "/__w/1/s/src/coreclr" "/__w/1/s/artifacts/obj/coreclr/linux.x64.Checked" x64 linux gcc Checked ""  -DCLR_CMAKE_PGO_INSTRUMENT=0 -DCLR_CMAKE_OPTDATA_PATH= -DCLR_CMAKE_PGO_OPTIMIZE=0 -DCLI_CMAKE_FALLBACK_OS="linux" -DFEATURE_DISTRO_AGNOSTIC_SSL=1 -DCDAC_BUILD_TOOL_BINARY_PATH=/__w/1/s/artifacts/bin/coreclr/linux.x64.Checked/cdac-build-tool/cdac-build-tool.dll -DCLR_DOTNET_HOST_PATH=/__w/1/s/.dotnet/dotnet -DCLR_DOTNET_RID=linux-x64 
  Not searching for unused variables given on the command line.
  -- The C compiler identification is GNU 15.1.0
  -- The CXX compiler identification is GNU 15.1.0

[jkoritzinsky]

/ba-g failures in unrelated legs