benchtests: Add random benchmark in bench-strchr.c

Add benchmark that randomizes whether return should be NULL or pointer
to CHAR. The rationale is on many architectures there is a choice
between a predicate execution option (i.e cmovcc on x86) or a branch.

On x86 the results for cmovcc vs branch are something along the lines
of the following:

perc-zero, Br On Result, Time Br / Time cmov
     0.10,            1,              ,0.983
     0.10,            0,              ,1.246
     0.25,            1,              ,1.035
     0.25,            0,              ,1.49
     0.33,            1,              ,1.016
     0.33,            0,              ,1.579
     0.50,            1,              ,1.228
     0.50,            0,              ,1.739
     0.66,            1,              ,1.039
     0.66,            0,              ,1.764
     0.75,            1,              ,0.996
     0.75,            0,              ,1.642
     0.90,            1,              ,1.071
     0.90,            0,              ,1.409
     1.00,            1,              ,0.937
     1.00,            0,              ,0.999
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>