GetPrerequisites: Optionally filter "objdump" output for speed

As dumpbin.exe is no longer reliable for gcc libraries on MinGW because
it crashes on many common libraries like libgcc_s and libgfortran it is
now necessary too resort to using objdump for DLL dependency walking.
Using objdump has a secondary problem in that it generates a lot of
output for large libraries and causes fixup_bundle() to take many
minutes to process what took fractions of a second with
"dumpbin.exe /dependents".

Add a 'grep' pre-filter in the execute_process() command pipeline to
reduce this output to a minimum for a several orders of magnitude speed
up.  If grep is not available just use the full output.

As there does not seem to be a reliable way of detecting MinGW, callers
of fixup_bundle() may have to set the variable gp_tool to "objdump" if
dumpbin.exe is installed on the build machine to stop it using the
broken MS dumpbin.exe for library dependency walking.

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