* [style] Use same style for cmake `set` command
* [build] Specify CMAKE_FIND_ROOT_PATH_MODE_PACKAGE in xxx.toolchain.cmake
* [build] Allow user passing args for cmake find cmds

Improves compatibility with Clang 11.

Also rename NCNN_SSE* options to NCNN_VSX_SSE* to avoid conflict between
x86 and POWER (went unnoticed before because x86 doesn't have an option
for toggling SSE 4.1).

Co-authored-by: Jeremy Rand <jeremyrand@danwin1210.de>

* Add POWER9 VSX toolchains

Translating x86_64 SSE to ppc64le VSX intrinsics yields a quite large
speedup on POWER9. See this article for background:

https://www.talospace.com/2019/07/easier-power-vectorizing-for-fun-and.html

* Add power9le docs

* power9le clang toolchain: Document Clang 13+ requirement

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Co-authored-by: Jeremy Rand <jeremyrand@danwin1210.de>