[PowerPC] Default ppc64 linux-gnu/freebsd to -fno-PIC

According to D53384, the default was switched from -fno-PIC to -fPIC to
work around a -fsanitize=leak bug on big-endian.

This gratuitous difference between little-endian and big-endian is
undesired, and not acceptable on powerpc64-unknown-freebsd.  If
-fsanitize=leak still has the problem, we should consider defaulting to
-fPIC/-fPIE only when -fsanitize=leak is specified (see SanitizerArgs::requiresPIE())

powerpc64-ibm-aix is unaffected: it still defaults to -fPIC.
powerpc64-linux-musl is unaffected (-fPIE since D39588): it still defaults to -fPIE.

Reviewed By: #powerpc, jhibbits

Differential Revision: https://reviews.llvm.org/D72363