Remove obsolete advice about linking with _DYNAMIC=0 (Linux)

Remove obsolete advice about linking with _DYNAMIC=0 (Linux)

Issue #196 (bdwgc).

The mentioned GCC option is not needed since gc-5.0alpha3 (when
_DYNAMIC symbol was declared as weak in dyn_load.c).
Even more, the advice is problematic for Alpine Linux (and probably
other systems using musl) as building a program with gcc -static
produces a binary containing a _DYNAMIC symbol with a non-zero address,
and forcing the address to 0 causes a crash during GC initialization.

* doc/README.linux: Remove note that static executable should be
linked with the gcc option "-Wl,-defsym,_DYNAMIC=0".