llama : use std::abs instead of abs

[kaetemi]

This replaces use of the global abs with std::abs.

The abs function declared outside of the std namespace is not required to have any of the C++ overloads defined. (Although it's also not precluded.)

Under Clang using the global abs with a larger integer type gives a truncation warning.

/home/kaetemi/llama.cpp/src/llama-quant.cpp:656:66: warning: absolute value function 'abs' given an argument of type 'const int64_t' (aka 'const long') but has parameter of type 'int' which may cause truncation of value [-Wabsolute-value]
  656 |                 gguf_set_val_u32(ctx_out.get(), o.key, (uint32_t)abs(o.val_i64));
      |                                                                  ^
/home/kaetemi/llama.cpp/src/llama-quant.cpp:656:66: note: use function 'std::abs' instead
  656 |                 gguf_set_val_u32(ctx_out.get(), o.key, (uint32_t)abs(o.val_i64));
      |                                                                  ^~~
      |                                                                  std::abs
1 warning generated.

Some standard libraries define the overloads globally, some don't. Recommended to always use the std:: functions to avoid anything unexpected.

See #15374 for further references.

This also was reported in the following issues:
#15044
#15254

This PR replaces the following incorrect PR:
#14209