[mips] Always use _LARGEFILE_SOURCE / _FILE_OFFSET_BITS for building MIPS 32-bit

When MIPS 32-bit compiler-rt is building on 32-bit host or using 32-bit
`DLLVM_HOST_TRIPLE` the `_LARGEFILE_SOURCE` and the `_FILE_OFFSET_BITS=64`
macros defined by statements from the `HandleLLVMOptions.cmake`. In case
of building 32-bit libraries on 64-bit host using default host triple
these macros are not defined. As a result assertions check a consistency
between the `struct_kernel_stat_sz` constant and the `struct_kernel_stat_sz`
start to fail.

To resolve this problem and enable building both 32/64-bit versions
of MIPS compiler-rt libraries on 64-bit host at once always explicitly
define the `_LARGEFILE_SOURCE` and the `_FILE_OFFSET_BITS=64` macros
for MIPS 32-bit.

llvm-svn: 360825