workaround broken hidden-visibility handling in pcc

with this change, pcc-built musl libc.so seems to work correctly. the
problem is that pcc generates GOT lookups for external-linkage symbols
even if they are hidden, rather than using GOT-relative addressing.
the entire reason we're using hidden visibility on the __libc object
is to make it accessible prior to relocations -- not to mention
inexpensive to access. unfortunately, the workaround makes it even
more expensive on pcc.

when the pcc issue is fixed, an appropriate version test should be
added so new pcc can use the much more efficient variant.

src/internal/libc.h

@@ -21,7 +21,7 @@ struct __libc {
 
 extern size_t __hwcap;
 
-#if !defined(__PIC__) || 100*__GNUC__+__GNUC_MINOR__ >= 303 || defined(__PCC__) || defined(__TINYC__)
+#if !defined(__PIC__) || (100*__GNUC__+__GNUC_MINOR__ >= 303 && !defined(__PCC__))
 
 #ifdef __PIC__
 #if __GNUC__ < 4