[coverage] Override -Wframe-larger-than

Bug: http://b/214221000
Bug: http://b/194128476

Adding `-mllvm -runtime-counter-relocation` for continuous coverage has
caused -Wframe-larger-than warnings when linking libart.  These warnings
need not be enforced on coverage builds, so override the flag.

Test: NATIVE_COVERAGE_PATHS=art CLANG_COVERAGE=true m libart;
      also go/abtd on failing target.
Change-Id: I8f5ac725ff5c674e6dd0831136d3d8b2e42b0428