boot: fix ia32 build with clang

boot: fix ia32 build with clang

After 668d915a4bea202e739a9bd3f895c7269f320659, the build fails:
[7/14] Linking target src/boot/systemd-bootia32.elf
FAILED: src/boot/systemd-bootia32.elf
clang  -o src/boot/systemd-bootia32.elf src/boot/systemd-bootia32.elf.p/boot.c.o src/boot/systemd-bootia32.elf.p/line-edit.c.o src/boot/systemd-bootia32.elf.p/bcd.c.o -Wl,--as-needed -Wl,--no-undefined -pie -fstack-protector src/boot/libefiia32.a -nostdlib -static-pie -Wl,--entry=efi_main -Wl,--fatal-warnings -Wl,-static,-pie,--no-dynamic-linker,-z,text -z common-page-size=4096 -z max-page-size=4096 -z noexecstack -z relro -z separate-code -Wl,-z,nopack-relative-relocs -fcf-protection=none -fno-asynchronous-unwind-tables -fno-exceptions -fno-unwind-tables -fno-sanitize=all -Wno-unused-command-line-argument -m32
/usr/sbin/ld: src/boot/libefiia32.a.p/console.c.o: in function `console_set_mode':
/home/zbyszek/src/systemd-work/build-clang/../src/boot/console.c:267:(.text+0xb8f): undefined reference to `__moddi3'
/usr/sbin/ld: src/boot/libefiia32.a.p/console.c.o: in function `get_auto_mode':
/home/zbyszek/src/systemd-work/build-clang/../src/boot/console.c:228:(.text+0xf18): undefined reference to `__udivdi3'
/usr/sbin/ld: src/boot/libefiia32.a.p/efi-string.c.o: in function `push_num':
/home/zbyszek/src/systemd-work/build-clang/../src/boot/efi-string.c:695:(.text+0x3c1a): undefined reference to `__moddi3'
/usr/sbin/ld: /home/zbyszek/src/systemd-work/build-clang/../src/boot/efi-string.c:696:(.text+0x3c54): undefined reference to `__divdi3'
/usr/sbin/ld: /home/zbyszek/src/systemd-work/build-clang/../src/boot/efi-string.c:702:(.text+0x3cda): undefined reference to `__umoddi3'
/usr/sbin/ld: /home/zbyszek/src/systemd-work/build-clang/../src/boot/efi-string.c:703:(.text+0x3d0a): undefined reference to `__udivdi3'
/usr/sbin/ld: src/boot/libefiia32.a.p/ticks.c.o: in function `time_usec':
/home/zbyszek/src/systemd-work/build-clang/../src/boot/ticks.c:110:(.text+0x9d): undefined reference to `__udivdi3'
/usr/sbin/ld: src/boot/libefiia32.a.p/ticks.c.o: in function `ticks_freq_arch':
/home/zbyszek/src/systemd-work/build-clang/../src/boot/ticks.c:46:(.text+0x394): undefined reference to `__udivdi3'
/usr/sbin/ld: /home/zbyszek/src/systemd-work/build-clang/../src/boot/ticks.c:49:(.text+0x3ce): undefined reference to `__udivdi3'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Restore the use of -lgcc for 32-bit builds.

(FWIW, neither --rtlib=compiler-rt nor --rtlib=libgcc help.)