Fix PR 11709: Change the definition of va_list to meet AAPCS requirement

AAPCS ABI Section 7.1.4 [1] specifies that va_list
should be defined as struct __va_list { void *__ap;};
And in C++, it is defined in namespace std.

[1] http://infocenter.arm.com/help/topic
         /com.arm.doc.ihi0042d/IHI0042D_aapcs.pdf

Patch by Weiming Zhao.

llvm-svn: 165609