Linpack: Use a fixed number of iterations for the linpack benchmark

Before, the benchmark used the executation time of running a loop containing
some function calls in the computation of how many iterations to run a later
loop.

Replace this by a fixed number of iterations.

git-svn-id: https://llvm.org/svn/llvm-project/test-suite/trunk@193930 91177308-0d34-0410-b5e6-96231b3b80d8