Simplify atoi*/strto* for signed integers.

Make the cost of strto<signed> closer to the cost of strto<unsigned>
by removing an `if` from the inner loop. Previously a signed conversion
cost 10ns more than an unsigned one.

After:

  BM_inttypes_strtoimax         81 ns         81 ns    8603362
  BM_inttypes_strtoumax         78 ns         78 ns    8967174
  BM_stdlib_strtol              81 ns         81 ns    8685537
  BM_stdlib_strtoll             81 ns         81 ns    8685481
  BM_stdlib_strtoul             78 ns         78 ns    8962569
  BM_stdlib_strtoull            78 ns         78 ns    8972023

Bug: N/A
Test: ran tests, benchmarks
Change-Id: I72dd5499427b6a940bd94c4d6f727f7efe134d7e