switching compiler to clang for benchmarks

[coveralls]

Pull Request Test Coverage Report for Build 4206533010

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.5%) to 53.579%

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Files with Coverage Reduction	New Missed Lines	%
src/EverCrypt_Curve25519.c	3	91.18%
<!--	Total:	3		-->

Totals
Change from base Build 4143236628:	0.5%
Covered Lines:	30536
Relevant Lines:	56992

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💛 - Coveralls