Fixed support for RTLD_NEXT in dlsym()

The previous implementation of this flag was broken--it behaved identically
to RTLD_DEFAULT.  This adds a proper implementation, which examines the address
of the calling function, and uses it to determine which library to use to begin
the symbol search process.

Change-Id: I2ad2b46363f68932af63a3828a22f9c7987eea67
diff --git a/linker/linker.h b/linker/linker.h
index 8cd56b0..68ac275 100644
--- a/linker/linker.h
+++ b/linker/linker.h
@@ -224,7 +224,7 @@
 soinfo *find_library(const char *name);
 unsigned unload_library(soinfo *si);
 Elf32_Sym *lookup_in_library(soinfo *si, const char *name);
-Elf32_Sym *lookup(const char *name, soinfo **found);
+Elf32_Sym *lookup(const char *name, soinfo **found, soinfo *start);
 soinfo *find_containing_library(void *addr);
 Elf32_Sym *find_containing_symbol(void *addr, soinfo *si);
 const char *linker_get_error(void);