Fix building DynamicLibrary.cpp with musl libc

[dim]

Summary

The workaround added in rL301240 for stderr/out/in symbols being both
macros and globals is only necessary for glibc, and it does not compile
with musl libc. Alpine Linux has had the following fix for it:

https://git.alpinelinux.org/cgit/aports/plain/main/llvm4/llvm-fix-DynamicLibrary-to-build-with-musl-libc.patch

Adapt the fix in our DynamicLibrary.inc for Unix.

[krytarowski]

/* Standard streams.  */
extern struct _IO_FILE *stdin;          /* Standard input stream.  */
extern struct _IO_FILE *stdout;         /* Standard output stream.  */
extern struct _IO_FILE *stderr;         /* Standard error output stream.  */
/* C89/C99 say they're macros.  Make them happy.  */
#define stdin stdin
#define stdout stdout
#define stderr stderr

Confirmed.

llvm/trunk/lib/Support/Unix/DynamicLibrary.inc

 #endif
 
 #undef EXPLICIT_SYMBOL
 
 // This macro returns the address of a well-known, explicit symbol
 #define EXPLICIT_SYMBOL(SYM) \
    if (!strcmp(SymbolName, #SYM)) return &SYM
 
-// On linux we have a weird situation. The stderr/out/in symbols are both
+// Under glibc we have a weird situation. The stderr/out/in symbols are both
 // macros and global variables because of standards requirements. So, we
 // boldly use the EXPLICIT_SYMBOL macro without checking for a #define first.
-#if defined(__linux__) and !defined(__ANDROID__)
+#if defined(__GLIBC__)
   {
     EXPLICIT_SYMBOL(stderr);
     EXPLICIT_SYMBOL(stdout);
     EXPLICIT_SYMBOL(stdin);
   }
 #else
   // For everything else, we want to check to make sure the symbol isn't defined
   // as a macro before using EXPLICIT_SYMBOL.