litecross: add some gcc options that should always be used with musl
libmpx is broken in all gcc versions that have it (not supported yet
by mcm/litecross, but will be).
ifunc is not supported by musl and won't be unless/until the contract
for resolvers can be specified clearly in a way that real-world usage
is safe and does not require lazy binding; having it enabled at gcc
level reportedly produces a broken libatomic.
explicitly enabling libstdcxx-time is needed to get libc clock_gettime
(with vdso support) to be used in libstdc++; it's disabled by default
because of glibc legacy practice of having clock_gettime in librt,
which depends on glibc's libpthread and undermines libstdc++'s
(invalid use of) weak references to pthread symbols.
Rich Felker