properly link libjulia to openlibm

From discussion with @Keno on slack, I believe this is the reasons the
tests added in #43870 are failing on linux32. (As seen in
https://build.julialang.org/#/builders/17/builds/1356/steps/5/logs/stdio
for example.)

Not sure if `RT_LIBS` is where this should go, so I'd appreciate any
feedback.

src/Makefile

@@ -149,7 +149,7 @@ OSLIBS += $(SRCDIR)/mach_dyld_atfork.tbd
 endif
 
 COMMON_LIBPATHS := -L$(build_libdir) -L$(build_shlibdir)
-RT_LIBS := $(LIBUV) $(LIBUTF8PROC) $(NO_WHOLE_ARCHIVE) $(LIBUNWIND) $(RT_LLVMLINK) $(OSLIBS)
+RT_LIBS := $(LIBM) $(LIBUV) $(LIBUTF8PROC) $(NO_WHOLE_ARCHIVE) $(LIBUNWIND) $(RT_LLVMLINK) $(OSLIBS)
 CG_LIBS := $(NO_WHOLE_ARCHIVE) $(LIBUV) $(LIBUNWIND) $(CG_LLVMLINK) $(OSLIBS)
 RT_DEBUG_LIBS := $(COMMON_LIBPATHS) $(WHOLE_ARCHIVE) $(BUILDDIR)/flisp/libflisp-debug.a $(WHOLE_ARCHIVE) $(BUILDDIR)/support/libsupport-debug.a -ljulia-debug $(RT_LIBS)
 CG_DEBUG_LIBS := $(COMMON_LIBPATHS) $(WHOLE_ARCHIVE) $(WHOLE_ARCHIVE) $(CG_LIBS) -ljulia-debug -ljulia-internal-debug