Workaround FreeBSD/clang/fmt bug

[phire]

/usr/home/buildbot/freebsd/pr-freebsd-x64/build/Source/UnitTests/Common/BitFieldTest.cpp:234:27: error: call to consteval function 'fmt::basic_format_string<char, BitField<0, 64, unsigned long, unsigned long> &>::basic_format_string<char [7], 0>' is not a constant expression
    EXPECT_EQ(fmt::format("{:02x}", object.full_u64),
                          ^
/usr/home/buildbot/freebsd/pr-freebsd-x64/build/Source/UnitTests/Common/BitFieldTest.cpp:234:27: note: undefined constructor 'basic_format_string<char [7], 0>' cannot be used in a constant expression
/usr/home/buildbot/freebsd/pr-freebsd-x64/build/Externals/fmt/include/fmt/core.h:3115:28: note: declared here
  FMT_CONSTEVAL FMT_INLINE basic_format_string(const S& s) : str_(s) {
                           ^
/usr/home/buildbot/freebsd/pr-freebsd-x64/build/Source/UnitTests/Common/BitFieldTest.cpp:236:27: error: call to consteval function 'fmt::basic_format_string<char, BitField<0, 64, long, long> &>::basic_format_string<char [7], 0>' is not a constant expression
    EXPECT_EQ(fmt::format("{:02x}", object.full_s64),

This appears to be a re-appearance of fmtlib/fmt#2455.

Except instead of being on AppleClang and MacOS, it's now on regular Clang and FreeBSD. Probably because the bug needs both clang13 and (some version of) llvm's libc++.

Our experiments showed that updating to Clang14 from ports appeared to fix it, but we would prefer to work with the stock freebsd 13.1. So, I've added a workaround to disable consteval on BSD with clang 13.

This is the same workaround that libfmt are already doing, except they detect AppleClang