cmake: set _LARGEFILE_SOURCE=1 to fix a cmake error
```
-- Performing additional configure checks with target flags: -mips32r2;-mabi=32;-D_LARGEFILE_SOURCE;-D_FILE_OFFSET_BITS=64
-- Performing Test COMPILER_RT_HAS_mips_FLOAT16
CMake Error: Parse error in command line argument: _LARGEFILE_SOURCE
Should be: VAR:type=value
```
See: https://buildd.debian.org/status/fetch.php?pkg=llvm-toolchain-16&arch=mips64el&ver=1%3A16.0.6-4&stamp=
1689111818&raw=0
Differential Revision: https://reviews.llvm.org/D155409