Set CPU_SUBTYPE_LIB64 in mach_header.

On Mac OS 10.5 and later, with X86_64 and outputting a dynamic executable,
ld64 set the CPU_SUBTYPE_LIB64 mask on the cpusubtype in the mach_header.

This adds the same functionality to lld.

rdar://problem/24507177

llvm-svn: 259826