[libc] Use nearest_integer instructions to improve expf performance.

Use nearest_integer instructions to improve expf performance.

Performance tests with CORE-MATH's perf tool:

Before the patch:
```
$ ./perf.sh expf
LIBC-location: /home/lnt/experiment/llvm-project/build/projects/libc/lib/libllvmlibc.a
GNU libc version: 2.31
GNU libc release: stable
CORE-MATH reciprocal throughput   : 9.860
System LIBC reciprocal throughput : 7.728
LIBC reciprocal throughput        : 12.363

$ ./perf.sh expf --latency
LIBC-location: /home/lnt/experiment/llvm-project/build/projects/libc/lib/libllvmlibc.a
GNU libc version: 2.31
GNU libc release: stable
CORE-MATH latency   : 42.802
System LIBC latency : 35.941
LIBC latency        : 49.808
```

After the patch:
```
$ ./perf.sh expf
LIBC-location: /home/lnt/experiment/llvm/llvm-project/build/projects/libc/lib/libllvmlibc.a
GNU libc version: 2.31
GNU libc release: stable
CORE-MATH reciprocal throughput   : 9.441
System LIBC reciprocal throughput : 7.382
LIBC reciprocal throughput        : 8.843

$ ./perf.sh expf --latency
LIBC-location: /home/lnt/experiment/llvm/llvm-project/build/projects/libc/lib/libllvmlibc.a
GNU libc version: 2.31
GNU libc release: stable
CORE-MATH latency   : 44.192
System LIBC latency : 37.693
LIBC latency        : 44.145
```

Reviewed By: zimmermann6

Differential Revision: https://reviews.llvm.org/D130498