[libc] Mark operator== const to avoid ambiguity in C++20. (#73954)

C++20 will automatically generate an operator== with reversed operand
order, which is ambiguous with the written operator== when one argument
is marked const and the other isn't.

This operator currently triggers -Wambiguous-reversed-operator at
several usage sites in libc/test/src/__support/CPP/bitset_test.cpp,
starting with line 153.