x86/boot: fix build with certain older gcc versions

x86/boot: fix build with certain older gcc versions

Despite all attempts so far (ending in commit fecf584294 ["Config.mk:
fix comment for debug option"] adjusting the respective comment),
Config.mk's debug= setting still affects the hypervisor build: CFLAGS
gets -g added there.

xen/arch/x86/boot/build32.mk includes that file, and hence inherits the
setting too. Some gcc versions take -g to create an .eh_frame section
despite -fno-asynchronous-unwind-tables (which instead one would expect
to produce .debug_frame).

In turn, commit 93c0c0287a ("x86/boot: create *.lnk files with linker
script") was - in my understanding - supposed to make sure .text is
first, but apparently it did also not really achieve that effect: Both
reloc.lnk and reloc.bin in the case here ended up with .eh_frame first,
which obviously rendered the whole final binary unusable.

Explicitly suppress generation of any kind of debug info when building
reloc.o.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>