cgo/out: include path to line macros to avoid build tmp directory being embedded

When the external linker is used the work directory of the compiler is
embedded into compressed sections of the LTO objects. Making them
unreproducible between builds.

The culprit seems to be fact that the line macros uses bare symbol
names, and GCC prepends each symbol with current directory when
compiling.

These files are included when building the final build id stamped into
the binary, thus they should be identical between builds.

Reproduction of the issue:

    $ CGO_CFLAGS='-flto' go build -work -trimpath -ldflags '-linkmode=external' main.go
    WORK=/tmp/go-build2408155205
    $ cd /tmp/go-build2408155205/b001
    $ objdump -h ./_x002.o | grep GoStringLen
      7 .gnu.lto__GoStringLen.0.6b7d5558e3553715 0000013b  0000000000000000  0000000000000000  0000009c  2**0
    $ objcopy ./_x002.o /dev/null --dump-section .gnu.lto__GoStringLen.0.6b7d5558e3553715=GoStringLen.zst
    $ unzstd ./GoStringLen.zst
    $ strings GoStringLen
    /tmp/go-build2408155205/b001
    cgo-builtin-prolog

Author: Foxboron

src/cmd/cgo/out.go

@@ -1471,7 +1471,7 @@ func (p *Package) cgoType(e ast.Expr) *Type {
 }
 
 const gccProlog = `
-#line 1 "cgo-gcc-prolog"
+#line 1 "/cgo-gcc-prolog"
 /*
   If x and y are not equal, the type will be invalid
   (have a negative array count) and an inscrutable error will come
@@ -1535,7 +1535,7 @@ const noTsanProlog = `
 // it is more important to avoid false positives, which reduce confidence
 // in the tool, than to avoid false negatives.
 const yesTsanProlog = `
-#line 1 "cgo-tsan-prolog"
+#line 1 "/cgo-tsan-prolog"
 #define CGO_NO_SANITIZE_THREAD __attribute__ ((no_sanitize_thread))
 
 long long _cgo_sync __attribute__ ((common));
@@ -1577,7 +1577,7 @@ extern void __msan_unpoison(const volatile void *, size_t);
 var msanProlog = noMsanProlog
 
 const builtinProlog = `
-#line 1 "cgo-builtin-prolog"
+#line 1 "/cgo-builtin-prolog"
 #include <stddef.h>
 
 /* Define intgo when compiling with GCC.  */
@@ -1766,7 +1766,7 @@ func (p *Package) cPrologGccgo() string {
 }
 
 const cPrologGccgo = `
-#line 1 "cgo-c-prolog-gccgo"
+#line 1 "/cgo-c-prolog-gccgo"
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
@@ -1873,7 +1873,7 @@ void localCgoCheckResult(Eface val) {
 // error if a Go file with a cgo comment #include's the export header
 // generated by a different package.
 const builtinExportProlog = `
-#line 1 "cgo-builtin-export-prolog"
+#line 1 "/cgo-builtin-export-prolog"
 
 #include <stddef.h>
 
@@ -1906,7 +1906,7 @@ func (p *Package) gccExportHeaderProlog() string {
 // difference, as Go code never wants to refer to the latter meaning.
 const gccExportHeaderProlog = `
 /* Start of boilerplate cgo prologue.  */
-#line 1 "cgo-gcc-export-header-prolog"
+#line 1 "/cgo-gcc-export-header-prolog"
 
 #ifndef GO_CGO_PROLOGUE_H
 #define GO_CGO_PROLOGUE_H
@@ -1968,7 +1968,7 @@ const gccExportHeaderEpilog = `
 // We use weak declarations, and test the addresses, so that this code
 // works with older versions of gccgo.
 const gccgoExportFileProlog = `
-#line 1 "cgo-gccgo-export-file-prolog"
+#line 1 "/cgo-gccgo-export-file-prolog"
 extern _Bool runtime_iscgo __attribute__ ((weak));
 
 static void GoInit(void) __attribute__ ((constructor));