Add a new virtual EmitStartOfAsmFile method to the AsmPrinter and use this

Add a new virtual EmitStartOfAsmFile method to the AsmPrinter and use this
to emit target-specific things at the beginning of the asm output.  This
fixes a problem for PPC, where the text sections are not being kept together
as expected.  The base class doInitialization code calls DW->BeginModule()
which emits a bunch of DWARF section directives.  The PPC doInitialization
code then emits all the TEXT section directives, with the intention that they
will be kept together. But as I understand it, the Darwin assembler treats
the default TEXT section as a special case and moves it to the beginning of
the file, which means that all those DWARF sections are in the middle of
the text.  With this change, the EmitStartOfAsmFile hook is called before
the DWARF section directives are emitted, so that all the PPC text section
directives come out right at the beginning of the file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@83176 91177308-0d34-0410-b5e6-96231b3b80d8