cmd/ld: add -extld and -extldflags options

Permits specifying the linker to use, and trailing flags to
pass to that linker, when linking in external mode.  External
mode linking is used when building a package that uses cgo, as
described in the cgo docs.

Also document -linkmode and -tmpdir.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8225043

src/cmd/5l/obj.c

@@ -114,6 +114,8 @@ main(int argc, char *argv[])
 	flagcount("a", "disassemble output", &debug['a']);
 	flagcount("c", "dump call graph", &debug['c']);
 	flagcount("d", "disable dynamic executable", &debug['d']);
+	flagstr("extld", "linker to run in external mode", &extld);
+	flagstr("extldflags", "flags for external linker", &extldflags);
 	flagcount("f", "ignore version mismatch", &debug['f']);
 	flagcount("g", "disable go package data checks", &debug['g']);
 	flagstr("k", "sym: set field tracking symbol", &tracksym);

src/cmd/6l/obj.c

@@ -107,6 +107,8 @@ main(int argc, char *argv[])
 	flagcount("a", "disassemble output", &debug['a']);
 	flagcount("c", "dump call graph", &debug['c']);
 	flagcount("d", "disable dynamic executable", &debug['d']);
+	flagstr("extld", "linker to run in external mode", &extld);
+	flagstr("extldflags", "flags for external linker", &extldflags);
 	flagcount("f", "ignore version mismatch", &debug['f']);
 	flagcount("g", "disable go package data checks", &debug['g']);
 	flagfn1("linkmode", "mode: set link mode (internal, external, auto)", setlinkmode);

src/cmd/8l/obj.c

@@ -113,6 +113,8 @@ main(int argc, char *argv[])
 	flagcount("a", "disassemble output", &debug['a']);
 	flagcount("c", "dump call graph", &debug['c']);
 	flagcount("d", "disable dynamic executable", &debug['d']);
+	flagstr("extld", "linker to run in external mode", &extld);
+	flagstr("extldflags", "flags for external linker", &extldflags);
 	flagcount("f", "ignore version mismatch", &debug['f']);
 	flagcount("g", "disable go package data checks", &debug['g']);
 	flagfn1("linkmode", "mode: set link mode (internal, external, auto)", setlinkmode);

src/cmd/ld/doc.go

@@ -71,5 +71,22 @@ Options new in this version:
 		NOTE: it only eliminates false positives caused by other function
 		calls, not false positives caused by dead temporaries stored in
 		the current function call.
+	-linkmode argument
+		Set the linkmode.  The argument must be one of
+		internal, external, or auto.  The default is auto.
+		This sets the linking mode as described in
+		../cgo/doc.go.
+	-tmpdir dir
+		Set the location to use for any temporary files.  The
+		default is a newly created directory that is removed
+		after the linker completes.  Temporary files are only
+		used in external linking mode.
+	-extld name
+		Set the name of the external linker to use in external
+		linking mode.  The default is "gcc".
+	-extldflags flags
+		Set space-separated trailing flags to pass to the
+		external linker in external linking mode.  The default
+		is to not pass any additional trailing flags.
 */
 package main

src/cmd/ld/lib.c

@@ -609,7 +609,7 @@ void
 hostlink(void)
 {
 	char *p, **argv;
-	int i, w, n, argc, len;
+	int c, i, w, n, argc, len;
 	Hostobj *h;
 	Biobuf *f;
 	static char buf[64<<10];
@@ -617,11 +617,22 @@ hostlink(void)
 	if(linkmode != LinkExternal || nerrors > 0)
 		return;
 
-	argv = malloc((10+nhostobj+nldflag)*sizeof argv[0]);
+	c = 0;
+	p = extldflags;
+	while(p != nil) {
+		while(*p == ' ')
+			p++;
+		if(*p == '\0')
+			break;
+		c++;
+		p = strchr(p + 1, ' ');
+	}
+
+	argv = malloc((10+nhostobj+nldflag+c)*sizeof argv[0]);
 	argc = 0;
-	// TODO: Add command-line flag to override gcc path and specify additional leading options.
-	// TODO: Add command-line flag to specify additional trailing options.
-	argv[argc++] = "gcc";
+	if(extld == nil)
+		extld = "gcc";
+	argv[argc++] = extld;
 	switch(thechar){
 	case '8':
 		argv[argc++] = "-m32";
@@ -679,6 +690,17 @@ hostlink(void)
 	argv[argc++] = smprint("%s/go.o", tmpdir);
 	for(i=0; i<nldflag; i++)
 		argv[argc++] = ldflag[i];
+
+	p = extldflags;
+	while(p != nil) {
+		while(*p == ' ')
+			*p++ = '\0';
+		if(*p == '\0')
+			break;
+		argv[argc++] = p;
+		p = strchr(p + 1, ' ');
+	}
+
 	argv[argc] = nil;
 
 	quotefmtinstall();

src/cmd/ld/lib.h

@@ -157,6 +157,8 @@ EXTERN	int flag_shared;
 EXTERN	char*	tracksym;
 EXTERN	char*	interpreter;
 EXTERN	char*	tmpdir;
+EXTERN	char*	extld;
+EXTERN	char*	extldflags;
 
 enum
 {