[libc] Use nearest_integer instructions to improve expm1f performance.

[libc] Use nearest_integer instructions to improve expm1f performance.

Use nearest_integer instructions to improve expf performance.

Performance tests with CORE-MATH's perf tool:

Before the patch:
```
$ ./perf.sh expm1f
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   : 10.096
System LIBC reciprocal throughput : 44.036
LIBC reciprocal throughput        : 11.575

$ ./perf.sh expm1f --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   : 42.239
System LIBC latency : 122.815
LIBC latency        : 50.122
```
After the patch:
```
$ ./perf.sh expm1f
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   : 10.046
System LIBC reciprocal throughput : 43.899
LIBC reciprocal throughput        : 9.179

$ ./perf.sh expm1f --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   : 42.078
System LIBC latency : 120.488
LIBC latency        : 41.528
```

Reviewed By: zimmermann6

Differential Revision: https://reviews.llvm.org/D130502