RtsFlags: Un-constify temporary buffer
RtsFlags: Un-constify temporary buffer

Otherwise we get a const-ness mismatch when we free the buffer, which
for some reason gcc 5.3 didn't notice.

(cherry picked from commit 378091c9ef6d25a076125f0b82dce1a155c0e8f0)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc