toolchain_mappings: remove unnecessary arch constraints

[jesec]

This change allows rules_foreign_cc to work on architectures other
than x86_64.

Bug: bazelbuild/bazel#11628
Test: manual. no longer fails with "ln: illegal option -- t" on darwin_arm64

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[jesec]

@googlebot I signed it!

[whs-dot-hk]

Look very legit to me the extra x86_64 condition was add during #196