[flang] avoid GCC < 8 compiler failure after D80794

[flang] avoid GCC < 8 compiler failure after D80794

Summary:
Patch D80794 remove the custom flags for release build for flang.
This leads to build failure with GCC < 8. This patch add upperbound check
in order to avoid the -Werror=array-bounds to trigger a build failure.

```
/home/4vn/versioning/llvm-project/flang/lib/Decimal/big-radix-floating-point.h:183:29: error: array subscript is above array bounds [-Werror=array-bounds]
           digit_[j] = digit_[j + remove];
                       ~~~~~~^
/home/4vn/versioning/llvm-project/flang/lib/Decimal/big-radix-floating-point.h:183:29: error: array subscript is above array bounds [-Werror=array-bounds]
           digit_[j] = digit_[j + remove];
                       ~~~~~~^
/home/4vn/versioning/llvm-project/flang/lib/Decimal/big-radix-floating-point.h:183:29: error: array subscript is above array bounds [-Werror=array-bounds]
           digit_[j] = digit_[j + remove];
                       ~~~~~~^
/home/4vn/versioning/llvm-project/flang/lib/Decimal/big-radix-floating-point.h:183:29: error: array subscript is above array bounds [-Werror=array-bounds]
           digit_[j] = digit_[j + remove];
                       ~~~~~~^
/home/4vn/versioning/llvm-project/flang/lib/Decimal/big-radix-floating-point.h:183:29: error: array subscript is above array bounds [-Werror=array-bounds]
           digit_[j] = digit_[j + remove];
```

```
/home/4vn/versioning/llvm-project/flang/include/flang/Evaluate/integer.h:809:28: error: array subscript is above array bounds [-Werror=array-bounds]
               xy += product[to];
                     ~~~~~~~^
/home/4vn/versioning/llvm-project/flang/include/flang/Evaluate/integer.h:810:22: error: array subscript is above array bounds [-Werror=array-bounds]
               product[to] = xy & partMask;
               ~~~~~~~^
/home/4vn/versioning/llvm-project/flang/include/flang/Evaluate/integer.h:809:28: error: array subscript is above array bounds [-Werror=array-bounds]
               xy += product[to];
                     ~~~~~~~^
```

Reviewers: DavidTruby, sscalpone, jdoerfert

Reviewed By: DavidTruby

Subscribers: llvm-commits

Tags: #llvm, #flang

Differential Revision: https://reviews.llvm.org/D81179