[build] Use separate code segment if supported by linker

Some versions of ld will complain that the automatically created (and
unused by our build process) ELF program headers include a "LOAD
segment with RWX permissions".

Silence this warning by adding "-z separate-code" to the linker
options, where supported.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

@@ -502,6 +502,13 @@ LDFLAGS		+= --gc-sections
 #
 LDFLAGS		+= -static
 
+# Use separate code segment if supported by linker
+#
+ZSC_TEST	= [ -z "`$(LD) -z separate-code --version 2>&1 > /dev/null`" ]
+ZSC_FLAGS	:= $(shell $(ZSC_TEST) && \
+		     $(ECHO) '-z separate-code -z max-page-size=4096')
+LDFLAGS		+= $(ZSC_FLAGS)
+
 # compiler.h is needed for our linking and debugging system
 #
 CFLAGS		+= -include include/compiler.h