Avoid recomputing COMPILER_/LINKER_ variables when set explicitly

[arichardson]

Summary

I noticed that when we build libraries for a different ABI (in CheriBSD) we
were calling ${XCC}/${LD} --version for every directory. It turns out that
this was caused by bsd.compat.mk explicitly setting (X_)COMPILER variables
for that build stage and this stops the _can_export logic from working.
To fix this, we change the check to only set _can_export=no if the variable
is set and it is set to a different value than the cached value.
This noticeably speeds up the tree walk while building compat libraries.

Obtained from: CheriBSD

Test Plan

Adding debug output shows fewer cc --version calls and we have been using this
patch for a long time in CheriBSD.

[emaste]

Seems OK to me although I hope @bdrewery can review/comment - perhaps allow a week.