add configure check for gnu linker hash style setting
add configure check for gnu linker hash style setting

some broken distro-provided toolchains have modified gcc to produce
only "gnu hash" dynamic hash table by default. as this is unsupported
by musl, that results in a non-working libc.so. we detect and switch
this on in configure rather than hard-coding it in the Makefile
because it's not supported by old binutils versions, but that might
not even be relevant since old binutils versions already fail from
-Bsymbolic-functions being missing. at some point I may review whether
this should just go in the Makefile...

chardi/musl,chardi/musl,chardi/musl