xen-locore: Silence DWARF2 section warning

Silence the "DWARF2 can only represent one section per compilation unit"
warning in amd64 GENERIC builds by disabling Clang's debuginfo generation for
this assembler file (-g0). The message is replaced by a warning from
ctfconvert that there is no debuginfo to convert (future work).

The file contains some metadata (several ELF notes) and some code. The code
does not appear to have anything that debuginfo would aid.

I looked at the generated debuginfo (readelf -w xen-locore.o) prior to this
change, and the metadata that would be disabled are things like associated
between binary offset and code line number (not especially useful with a
disassembler), and label metadata for the entry points (not especially useful
as this is already in the symbol table).

Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D24384