tu: support l1 dcache size on musl
musl doesn't support _SC_LEVEL1_DCACHE_LINESIZE
so calculate the size manually like on Bionic.
Note that the change would result in different behavior on Bionic when _SC_LEVEL1_DCACHE_LINESIZE
is not defined (the new version would manually calculate the cache size while the old version would use 0). I'm not sure if this could ever happen though.
Also note that I've kept the DETECT_ARCH_*
checks to keep the code equivalent but I think these might not actually be necessary.