Log Message:
vmips: update to 1.5.1.

Based on PR 53556 by Brian R. Gaeke.

vmips-1.5.1 was released on 20 August 2018.  User-visible changes
in version 1.5 (since version 1.5.1):

   * Bug fixes:

   * A pair of time calculations in the clock device were revised to
     avoid triggering compile-time errors in newer C++11-compliant
     compilers.

Improve 'make test'; fails because it needs a mips toolchain.