Repository: tensorflow
Issue: Remove GCC_HOST_COMPILER_PREFIX as it may be out of sync with GCC_HOST_COMPILER_PATH
Labels: stat:awaiting tensorflower,type:build/install,subtype: ubuntu/linux,TF 2.2

**System information**
- OS Platform and Distribution (e.g., Linux Ubuntu 16.04): RHEL 7.5
- TensorFlow installed from (source or binary): source
- TensorFlow version: 2.2.0
- Python version: 3.7.4
- Bazel version (if compiling from source): 2.0.0
- GCC/Compiler version (if compiling from source): 7.3.0


**Describe the problem**

The file https://github.com/tensorflow/tensorflow/blob/1588f45ee56860d247a1c26ea228cb3721b4bf1b/third_party/gpus/cuda_configure.bzl has a documented environment variable `GCC_HOST_COMPILER_PATH` to set the path (or name) of a GCC host compiler. However it also uses an undocumented variable `GCC_HOST_COMPILER_PREFIX` to get the folder where the gcc binary resides (guessing from name) defaulting to `/usr/bin` if it isn't set:
https://github.com/tensorflow/tensorflow/blob/1588f45ee56860d247a1c26ea228cb3721b4bf1b/third_party/gpus/cuda_configure.bzl#L1028-L1030

I have 2 problems with that:
- It is undocumented and hence hard to set right if you don't know for sure what it is
- The default is wrong when the (documented!) `GCC_HOST_COMPILER_PATH`  is used

This leads to issues such as 

```
 external/local_config_cuda/crosstool/clang/bin/crosstool_wrapper_driver_is_not_gcc -o bazel-out/k8-opt/bin/external/protobuf_archive/js_embed -Wl,-no-as-needed -pie -Wl,-z,relro,-z,now '-Wl,--build-id=md5' '-Wl,--hash-style=gnu' -no-canonical-prefixes -B/usr/bin -Wl,--gc-sections -Wl,@bazel-out/k8-opt/bin/external/protobuf_archive/js_embed-2.params)
/usr/bin/ld.gold: error: /software/software/GCCcore/7.3.0/lib/gcc/x86_64-pc-linux-gnu/7.3.0/crtbeginS.o: unsupported reloc 42 against global symbol _ITM_deregisterTMCloneTable
/usr/bin/ld.gold: error: /software/software/GCCcore/7.3.0/lib/gcc/x86_64-pc-linux-gnu/7.3.0/crtbeginS.o: unsupported reloc 42 against global symbol _ITM_registerTMCloneTable
/usr/bin/ld.gold: error: bazel-out/k8-opt/bin/external/protobuf_archive/_objs/js_embed/embed.o: unsupported reloc 42 against global symbol std::ios_base::Init::~Init()
/software/software/GCCcore/7.3.0/lib/gcc/x86_64-pc-linux-gnu/7.3.0/crtbeginS.o(.text+0x1a): error: unsupported reloc 42
/software/software/GCCcore/7.3.0/lib/gcc/x86_64-pc-linux-gnu/7.3.0/crtbeginS.o(.text+0x6b): error: unsupported reloc 42
bazel-out/k8-opt/bin/external/protobuf_archive/_objs/js_embed/embed.o:embed.cc:function _GLOBAL__sub_I_main: error: unsupported reloc 42
collect2: error: ld returned 1 exit status
```

As reported at https://github.com/easybuilders/easybuild-easyconfigs/pull/7800#issuecomment-471447493

I hence propose to either completely remove that variable in favor of deriving its value from `GCC_HOST_COMPILER_PATH` or properly documenting it with a better default.

It does not seem to be required at all so it's likely best to just remove it. This should have been done by #34218 but for some reason that merge was reverted with a very misleading commit title: https://github.com/tensorflow/tensorflow/commit/f0571998d0195b5b243cf409a64d5fa17bd44d43

@gunan @mihaimaruseac please take a look what went wrong