um: Switch to large mcmodel on x86_64

x86_64 UML is unable to load modules if more than 504MiB
of memory are used.
This happens because on x86_64 the UML process has a quite high
start address (typically around 0x6000000).
If UML's memory is larger than 504MiB VMALLOC_START happens to be after
0x8000000. This is no problem unless one loads a module which was built
with R_X86_64_32S relocations.
Symbols with a location > 0x8000000 cannot be used with R_X86_64_32S

To deal with this x86_64 UML has to be compiled with -mcmodel=large
such that no R_X86_64_32S relocations are used.

Signed-off-by: Richard Weinberger <richard@nod.at>
Reported-by: 전하늘 <allskyee@gmail.com>