x86: generate labels at the beginning of unlikely sub-sections

x86: generate labels at the beginning of unlikely sub-sections

This is to limit symbol table growth, which would be quite a bit worse
if we went with the "label every unlikely sub-section contribution"
approach proposed previously.

Older gas doesn't support quoted symbols, yet the result looks quite
bit better that way. Hence two variants get introduced, one using
proper path names (including slashes and dashes) and one using path
names after converting them to valid symbol names (slashes and dashes
replaced).

As a secondary adjustment also change the section name used with Clang.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper@citrix.com>