When building musl with pnacl-clang compiler I found a bug: crt/rcrt1.c
defines the SHARED macro which enables the conditional block in
src/ldso/dlstart.c, this breaks for us because pnacl-clang doesn't support
shared libraries nor inline assembly. If my understanding is correct, the
SHARED macro should be (and already is) defined only in CFLAGS_ALL_SHARED.
The attached patch fixes that.