x86/lib: Make get/put_user() exception handling a visible symbol

The .L-prefixed exception handling symbols of get_user() and put_user()
do get discarded from the symbol table of the final kernel image.

This confuses tools which parse that symbol table and try to map the
chunk of code to a symbol. And, in general, from toolchain perspective,
it is a good practice to have all code belong to a symbol, and the
correct one at that.

  ( Currently, objdump displays that exception handling chunk as part
    of the previous symbol which is a "fallback" of sorts and not
    correct. )

While at it, rename them to something more descriptive.

  [ bp: Rewrite commit message, rename symbols. ]

Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230525184244.2311-1-namit@vmware.com