[Squash On Rebase] Switch BaseTools to GCC

[Aaron (apop5)]

Description

GCC5 has been deprecated in EDK2, replaced by just GCC.

Switching release-basetools to use GCC TOOL_CHAIN_TAG

• Impacts functionality?
• Impacts security?
• Breaking change?
• Includes tests?
• Includes documentation?

How This Was Tested

Forked Repo run of release-basetools github action.

Integration Instructions

No integration necessary

[mu-automation]

⏩ QEMU Validation Skipped

The PR was merged before validation completed.

This comment was automatically generated by the Mu QEMU PR Validation workflow.

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (release/202608@a030777). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release/202608   #1900   +/-   ##
================================================
  Coverage                  ?   5.10%           
================================================
  Files                     ?     713           
  Lines                     ?   81761           
  Branches                  ?       0           
================================================
  Hits                      ?    4170           
  Misses                    ?   77591           
  Partials                  ?       0           

Flag	Coverage Δ
FmpDevicePkg	8.54% <ø> (?)
MdePkg	4.84% <ø> (?)
PolicyServicePkg	28.75% <ø> (?)
StandaloneMmPkg	0.47% <ø> (?)
UefiCpuPkg	4.17% <ø> (?)
UnitTestFrameworkPkg	16.29% <ø> (?)

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