[mips][sanitizer_common] Don't use `ld` in internal_clone() on 32-bit MIPS.

Summary:
On a 32-bit MIPS, the `ld` instruction does not exist. However, GAS has an `ld`
macro that expands to a pair of `lw` instructions which load to a pair of
registers (reg, and reg+1). This macro is not available in the Integrated
Assembler and its use causes -fintegrated-as builds to fail. Even if it were
available, the behaviour on 32-bit MIPS would be incorrect since the current
usage of `ld` causes the code to clobber $5 (which is supposed to hold
child_stack). It also clobbers $k0 which is reserved for kernel use.

Aside from enabling builds with the integrated assembler, there is no functional
change since internal_clone() is only used by StopTheWorld() which is only used
by 64-bit sanitizers.

Reviewers: kcc, sagar

Subscribers: mohit.bhakkad, jaydeep, sagar, llvm-commits

Differential Revision: http://reviews.llvm.org/D18753

llvm-svn: 269297