[ELF] Synthetic symbol definitions for ARM static linking

    
When static linking in ARM (like Mips) __tls_get_addr is defined by
the library so we should not define it as a synthetic.
    
We also need to add __exidx_start and __exidx_end for the .ARM.exidx
section as the static libc library startup code is expecting them to
be defined by the default linker script for static linking on ARM.

Differential revision: https://reviews.llvm.org/D25978

llvm-svn: 285279