Set the thumb flag for thumb symbols coming from an ELF file

Without setting the flag there is no way to determine if a symbol
points to an arm or to a thumb function as the LSB of the address
masked out in all getter function.

Note: Currently the thumb flag is only used for MachO files so
adding a test to this change is not possible. It will be used
by the upcoming fix for llvm-objdump for disassembling thumb
functions what is easily testable.

Differential revision: http://reviews.llvm.org/D17956

llvm-svn: 265387