pref: add -m32, -m64 command-line options (#7011)

cmd/v/help/build-c.txt

@@ -44,6 +44,9 @@ These build flags are enabled on `build` and `run` as long as the backend is set
       List of OS supported by V: `linux`, `windows`, `ios`, `mac`, `freebsd`, `openbsd`,
                                  `netbsd`, `dragonfly`, `solaris`, `android` and `haiku`.
 
+   -m32, -m64
+      Specify whether 32-bit or 64-bit machine code is generated.
+
    -sanitize
       Pass flags related to sanitization to the C compiler.
 

vlib/v/pref/pref.v

@@ -85,6 +85,7 @@ pub mut:
 	// For example, passing -cflags -Os will cause the C compiler to optimize the generated binaries for size.
 	// You could pass several -cflags XXX arguments. They will be merged with each other.
 	// You can also quote several options at the same time: -cflags '-Os -fno-inline-small-functions'.
+	m64                 bool // true = generate 64-bit code, defaults to x64
 	ccompiler           string // the name of the C compiler used
 	ccompiler_type      CompilerType // the type of the C compiler used
 	third_party_option  string
@@ -138,6 +139,9 @@ pub mut:
 
 pub fn parse_args(args []string) (&Preferences, string) {
 	mut res := &Preferences{}
+	$if x64 {
+		res.m64 = true // follow V model by default
+	}
 	mut command := ''
 	mut command_pos := 0
 	// for i, arg in args {
@@ -239,6 +243,10 @@ pub fn parse_args(args []string) (&Preferences, string) {
 			'-color' {
 				res.use_color = .always
 			}
+			'-m32', '-m64' {
+				res.m64 = arg[2] == `6`
+				res.cflags += ' $arg'
+			}
 			'-nocolor' {
 				res.use_color = .never
 			}