cross-compilation incorrectly searches the host for cmake find_* commands

For #2277 (closed) my arm cross-compile builds were claiming that zlib was available, but it was the host x86 zlib, and:

arm-debug-internal-64: **** 6 build errors ****
	/work/dr/git/src/clients/drcachesim/reader/compressed_file_reader.h:38:18: fatal error: zlib.h: No such file or directory

We should revisit the CMAKE_FIND_ROOT_PATH settings in our toolchain files: it looks like they are set to search the host dirs for find_*(), which does not seem the right thing to do.