Profiling tests: Endianess XFAIL for powerpc- (32-bit)

Add powerpc- (32-bit) as XFAIL for tests that are documented either in-
line or via commit messages as expected to fail on big-endian systems.

Tests not documented in-line are documented in commit messages as
follows:
r211172 - test/tools/llvm-cov/llvm-cov.test
r247920 - test/Transforms/SampleProfile/gcc-simple.ll

llvm-svn: 322114