silo: Add 64-bit support, v2

[John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>]

Replaced "LD=ld -m elf64_sparc" with "LD=ld" which essentially
reverts c836dbda which hard-wired the build to 32 bits. Since
both 32- and 64-bit builds are supported these days, this should
no longer be necessary.

Adrian