Always indirect branch to __libc_start_main via GOT

Always indirect branch to __libc_start_main via GOT

Since __libc_start_main in libc.so is called very early, lazy binding
isn't relevant.  Always call __libc_start_main with indirect branch via
GOT to avoid extra branch to PLT slot.  In case of static executable,
ld in binutils 2.26 or above can convert indirect branch into direct
branch:

0000000000400a80 <_start>:
  400a80:       31 ed                   xor    %ebp,%ebp
  400a82:       49 89 d1                mov    %rdx,%r9
  400a85:       5e                      pop    %rsi
  400a86:       48 89 e2                mov    %rsp,%rdx
  400a89:       48 83 e4 f0             and    $0xfffffffffffffff0,%rsp
  400a8d:       50                      push   %rax
  400a8e:       54                      push   %rsp
  400a8f:       49 c7 c0 20 1b 40 00    mov    $0x401b20,%r8
  400a96:       48 c7 c1 90 1a 40 00    mov    $0x401a90,%rcx
  400a9d:       48 c7 c7 c0 03 40 00    mov    $0x4003c0,%rdi
  400aa4:       67 e8 96 09 00 00       addr32 callq 401440 <__libc_start_main>
  400aaa:       f4                      hlt

	* sysdeps/x86_64/start.S (_start): Always indirect branch to
	__libc_start_main via GOT.