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Only compute the X_COMPILER_*/X_LINKER_* variables when needed

Description

Only compute the X_COMPILER_*/X_LINKER_* variables when needed

When building CheriBSD we have to set XLD/XCC/XCFLAGS on the command line.
This triggers the $XCC != $CC case in bsd.compiler.mk (and the same for LD
in bsd.linker.mk) which causes it to call ${XCC} --version and
${XLD} --version (plus various awk+sed+echo calls) in every subdirectory.
For incremental builds and stages that only walk the source tree this is
often the majority of the time spent in that directory.

By only computing the value of the X_COMPILER_*/X_LINKER_* variables if
_WANT_TOOLCHAIN_CROSS_VARS is set we can reduce the number of cc/ld calls
to once per build stage instead of once per recursive make.

With this change (and no changes to the sources) the make includes stage
now takes 28 seconds at -j1 instead of 86 seconds.

Approved By: brooks (mentor)
Differential Revision: https://reviews.freebsd.org/D17046

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