pcre and pcre2: fix instruction cache flush bug on ppc/ppc64/ppc64le.

[jhamby]

GCC's version of __builtin___clear_cache() is a no-op on PowerPC,
which causes the unit tests to fail on at least PowerPC 970 and 7447A,
and is technically incorrect. Change the code to use an improved version
of the existing ppc_cache_flush() function that wasn't being called.

Testing the changes

• I tested the changes in this PR: YES
• I tested on a Quad G5 2.5 GHz and a Mac mini G4. Both failed "make check" without the patch, and both succeeded with the patch.

Local build testing

• I built this PR locally for my native architectures, ppc64-glibc and ppc-glibc.
• This patch needs to be tested on some POWER5 or newer systems, just to make sure the "instruction cache snoop" fast path also works.

[q66]

i will take a look at this later, thanks