cmd/link: fix cgo on riscv64 when building with gcc-15

It's not currently possible to build cgo programs that are partially
compiled with gcc-15 on riscv64 using the internal linker. There are
two reasons for this.

1. When gcc-15 compiles _cgo_export.c, which contains no actual code,
   for a riscv64 target, it emits a label in the .text section called
   .Letext0. This label is referred to by another section, .debug_line,
   and an entry is generated in the symbol table for it. The Go linker
   panics when processing the .Letext0 symbol in _cgo_export.o, as it
   occurs in an empty section.
2. GCC-15 is generating additional debug symbols with the .LVUS
   prefix, e.g., .LVUS33, that need to be ignored.

We fix the issue by removing the check in
cmd/link/internal/loader/loader.go that panics if we encounter a
symbol in an empty section (the comments preceding this check suggest
it's safe to remove it) and by adding .LVUS to the list of symbol
prefixes to ignore.

Fixes #72840

Change-Id: I00658b6bdd01606dde1581b5bc2f42edfc37de82
Reviewed-on: https://go-review.googlesource.com/c/go/+/668276
Auto-Submit: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Joel Sing <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Reviewed-by: Meng Zhuo <[email protected]>

Author: markdryan

src/cmd/link/internal/loadelf/ldelf.go

@@ -609,7 +609,7 @@ func Load(l *loader.Loader, arch *sys.Arch, localSymVersion int, f *bio.Reader,
 				continue
 			}
 
-			if strings.HasPrefix(elfsym.name, ".LASF") || strings.HasPrefix(elfsym.name, ".LLRL") || strings.HasPrefix(elfsym.name, ".LLST") {
+			if strings.HasPrefix(elfsym.name, ".LASF") || strings.HasPrefix(elfsym.name, ".LLRL") || strings.HasPrefix(elfsym.name, ".LLST") || strings.HasPrefix(elfsym.name, ".LVUS") {
 				// gcc on s390x and riscv64 does this.
 				continue
 			}

src/cmd/link/internal/loader/loader.go

@@ -1719,14 +1719,6 @@ func (l *Loader) GetVarDwarfAuxSym(i Sym) Sym {
 // expected to have the actual content/payload) and then a set of
 // interior loader.Sym's that point into a portion of the container.
 func (l *Loader) AddInteriorSym(container Sym, interior Sym) {
-	// Container symbols are expected to have content/data.
-	// NB: this restriction may turn out to be too strict (it's possible
-	// to imagine a zero-sized container with an interior symbol pointing
-	// into it); it's ok to relax or remove it if we counter an
-	// oddball host object that triggers this.
-	if l.SymSize(container) == 0 && len(l.Data(container)) == 0 {
-		panic("unexpected empty container symbol")
-	}
 	// The interior symbols for a container are not expected to have
 	// content/data or relocations.
 	if len(l.Data(interior)) != 0 {