Allow overriding the tool used for stripping binaries

Since the make variable STRIP is already used for other purposes, this
uses STRIPBIN (which is also used for the same purpose by install(1).
This allows using LLVM objcopy to strip binaries instead of the in-tree
elftoolchain objcopy. We make use of this in CheriBSD since passing
binaries generated by our toolchain to elftoolchain strip sometimes results
in assertion failures.

This allows working around https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=248516
by specifying STRIPBIN=/path/to/llvm-strip

Obtained from: CheriBSD
Reviewed By: emaste, brooks
Differential Revision: https://reviews.freebsd.org/D25988