[compiler-rt] Fix extendhfxf2 test (PR #117665)

[Rainer Orth via llvm-commits]

rorth wrote:

Things are similar on Solaris/amd64:
```
  Builtins-i386-sunos :: divdf3_test.c
  Builtins-i386-sunos :: divsc3_test.c
  Builtins-i386-sunos :: divsf3_test.c
  Builtins-i386-sunos :: extendhfsf2_test.c
  Builtins-i386-sunos :: extendhfxf2_test.c
  Builtins-i386-sunos :: truncdfhf2_test.c
  Builtins-i386-sunos :: truncdfsf2_test.c
  Builtins-i386-sunos :: truncsfhf2_test.c
```
AFAICS the failures are all for the same reason:
```
In file included from /vol/llvm/src/llvm-project/local/compiler-rt/test/builtins/Unit/divdf3_test.c:7:
/vol/llvm/src/llvm-project/local/compiler-rt/test/builtins/Unit/fp_test.h:235:5: error: use of undeclared identifier '__uint128_t'
  235 |     __uint128_t x = ((__uint128_t)hi << 64) + lo;
      |     ^
```
`__uint128_t` etc. isn't available on 32-bit x86 (not only on Solaris, same on Linux/x86_64 `-m32`).

https://github.com/llvm/llvm-project/pull/117665