[libc++] fix test for unsigned char

On some systems char is unsigned.
If that is the case, we will now
test signed char twice in std::abs.
NFC. Fixes the build bots.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@369413 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed