build-llvm: Add an option --use-linker=

This allows overriding which linker to use.

Currently, building LLVM on Ubuntu 25.10 with these scripts fails,
if LLD is available. This, because the GCC on that distribution
produces object files that LLD (and gold) rejects during linking - see
https://github.com/llvm/llvm-project/issues/156667 for details.

By building with --use-linker=bfd, linking does succeed in that
configuration.

Also pass the same option through from build-all.sh.

build-all.sh

@@ -24,7 +24,7 @@ HOST_ARGS=""
 
 while [ $# -gt 0 ]; do
     case "$1" in
-    --enable-asserts|--disable-dylib|--with-clang|--thinlto)
+    --enable-asserts|--disable-dylib|--with-clang|--thinlto|--use-linker=*)
         LLVM_ARGS="$LLVM_ARGS $1"
         ;;
     --host-clang|--host-clang=*)
@@ -120,7 +120,7 @@ while [ $# -gt 0 ]; do
     shift
 done
 if [ -z "$PREFIX" ]; then
-    echo "$0 [--host-clang[=clang]] [--enable-asserts] [--disable-dylib] [--with-clang] [--thinlto] [--full-llvm] [--disable-lldb] [--disable-lldb-mi] [--disable-clang-tools-extra] [--host=triple] [--with-default-win32-winnt=0x601] [--with-default-msvcrt=ucrt] [--enable-cfguard|--disable-cfguard] [--no-runtimes] [--llvm-only] [--no-tools] [--wipe-runtimes] [--clean-runtimes] [--stage1] [--profile[=type]] [--pgo[=profile]] [--full-pgo[=type]] dest [pgo-dest]"
+    echo "$0 [--host-clang[=clang]] [--enable-asserts] [--disable-dylib] [--with-clang] [--use-linker=linker] [--thinlto] [--full-llvm] [--disable-lldb] [--disable-lldb-mi] [--disable-clang-tools-extra] [--host=triple] [--with-default-win32-winnt=0x601] [--with-default-msvcrt=ucrt] [--enable-cfguard|--disable-cfguard] [--no-runtimes] [--llvm-only] [--no-tools] [--wipe-runtimes] [--clean-runtimes] [--stage1] [--profile[=type]] [--pgo[=profile]] [--full-pgo[=type]] dest [pgo-dest]"
     exit 1
 fi
 if [ -n "$PREFIX_PGO" ] && [ -z "$PGO" ] && [ -z "$FULL_PGO" ]; then