the initializer in libgcc uses __getauxval which is not
available on non-gnu targets so outlining atomics is
ineffective.

change the runtime lse check in libgcc such that non-glibc
targets can implement it too (e.g. calling __getauxval via
a weak reference and no #ifdef __gnu_linux__ check allows
a libc to implement it later, unfortunately a non-linux os
may not have the same hwcap mechanism so a more generic
libc<->libgcc abi would be better).