[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.

For BIOS builds, where the prefix will generally require writable
access to its own (tiny) code segment, simply inhibit the warning
completely via "--no-warn-rwx-segments".

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	= $(LD) -z separate-code --version 2>&1 > /dev/null
+ZSC_FLAGS	:= $(shell [ -z "`$(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

src/arch/x86/Makefile.pcbios

@@ -13,6 +13,13 @@ LDSCRIPT_PREFIX	= arch/x86/scripts/prefixonly.lds
 #
 LDFLAGS		+= -N --no-check-sections
 
+# Do not warn about RWX segments (required by most prefixes)
+#
+WRWX_TEST	= $(LD) --warn-rwx-segments --version 2>&1 > /dev/null
+WRWX_FLAGS	:= $(shell [ -z "`$(WRWX_TEST)`" ] && \
+		     $(ECHO) '--no-warn-rwx-segments')
+LDFLAGS		+= $(WRWX_FLAGS)
+
 # Media types.
 #
 MEDIA		+= rom