[libc] Make expm1f correctly rounded when the targets have no FMA instructions.

Add another exceptional value and fix the case when |x| is small.

Performance tests with CORE-MATH project scripts:
With FMA instructions on Ryzen 1700:
```
$ ./perf.sh expm1f
LIBC-location: /home/lnt/experiment/llvm/llvm-project/build/projects/libc/lib/libllvmlibc.a
CORE-MATH reciprocal throughput   : 15.362
System LIBC reciprocal throughput : 53.194
LIBC reciprocal throughput        : 14.595
$ ./perf.sh expm1f --latency
LIBC-location: /home/lnt/experiment/llvm/llvm-project/build/projects/libc/lib/libllvmlibc.a
CORE-MATH latency   : 57.755
System LIBC latency : 147.020
LIBC latency        : 60.269
```
Without FMA instructions:
```
$ ./perf.sh expm1f
LIBC-location: /home/lnt/experiment/llvm/llvm-project/build/projects/libc/lib/libllvmlibc.a
CORE-MATH reciprocal throughput   : 15.362
System LIBC reciprocal throughput : 53.300
LIBC reciprocal throughput        : 18.020
$ ./perf.sh expm1f --latency
LIBC-location: /home/lnt/experiment/llvm/llvm-project/build/projects/libc/lib/libllvmlibc.a
CORE-MATH latency   : 57.758
System LIBC latency : 147.025
LIBC latency        : 70.304
```

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D123440